A character's position amounts to two things: their whereabouts, and who they're contending with in melee.

Whereabouts 

Each character has a general, relative position on the battlefield, defined by their wereabouts. 

Melee

You you are right next to another combatant, you are considered to be in Melee with them. Generally, a character can be in Melee with a maximum number of other combatants equal to their Size, though these numbers can be affected by environmental factors--fighting in a chokepoint like a doorway or narrow alley might lower this number to two or three, while swarming creatures or panicked mobs may allow more combatants to engage.

The maximum number of combatants who can be engaged in a collective melee is equal to the Size of the largest creature in that melee, modified by environmental factors. 

When two allies are in Melee with the same enemy, they are considered to be flanking with each other. While you are flanking, the enemy you are both in Melee with gains the Flat-Footed condition. You do not grant flanking if you are not wielding a melee weapon (or have the Combat Martial Arts feat), or you are under any effect that prevents you from attacking or making reactions (such as the Slowed or Immobilized conditions)

Exiting Melee without taking a Guarded Step or Withdraw action provokes an Attack of Opportunity from all foes you are in Melee with.

Close

Characters who are Close to you can be reached in a single Move action (or two Move actions if you're in difficult terrain.)

Close refers to an area roughly 30 feet or 9 metres around your character.

Range 

Range is a scaling condition that applies to creatures. To reach a target at range, you must make a number of Move actions equal to 1+their Range (A number of moves to enter into close, and then one move action to enter Melee). 

Each Range indicates a target up to 30 feet or 9 metres beyond the previous range increment. So "Range 1" indicates creatures that are beyond 30 feet/9 metres from you, but are within 60 feet or 18 metres. Range 2 indicates things beyond 60 feet/18 metres but within 90 feet or 27 metres. And so on. 

Ranged weapons have a short, medium and long-range value. Shooting at a target beyond the short-range imposes a circumstance penalty of -2 to hit. Shooting at a target beyond the medium range imposes a circumstance penalty of -4 to hit. You cannot fire your weapon beyond the weapon's long range. 

For example, an Assault Riffle has range values of 2, 4 and 6. That means you can fire at a target with 2 Range with no penalties, but cannot hit targets beyond 6 Range. 

When you use Charge action, you take 3 actions to move up to 2 Range, enter Melee, and Attack.

Charge (3 Actions) ⬤ ⬤ ⬤

Charging takes 3 actions, and allows you to move up 2 Range, enter into Melee with a target, and make a melee attack against the target at the end of the movement. You can draw a melee weapon during a charge attack if you possess at least 1 rank in Melee Weapons.

Charging carries tight restrictions on how you can move. You must move at least 1 Range, and all movement must be directly toward the designated target. You must have a clear path toward the target. If there is any obstacle, difficult terrain, or creature in the way, you cannot charge. If you don’t have a line of sight to the opponent at the start of your turn, you can’t charge that opponent. 

Attacking on a Charge: After moving, you can make a single melee attack. You gain a  +2 bonus to the attack roll and a -2 penalty to your Armour Defence until the start of your next turn.  

When you use a Withdraw action, you can move up to 2 Range without provoking attacks of opportunity, so long as that movement is away from enemies. 

Withdraw (3 Actions) ⬤ ⬤ ⬤ 

Withdrawing from melee combat takes 3 actions. When you withdraw, you can move up to 2 Range. You are not considered threatened by targets you start off in Melee with, and therefore visible enemies don’t get to make attacks of opportunity against you when you move from that square. Unseen enemies still get attacks of opportunity against you, and you can’t withdraw from combat if you’re blinded and have no other precise senses (such as blindsight). 

When you use a Run action, you can move up to 4 Range in one round. 

Run (3 Actions) ⬤ ⬤ ⬤

By spending 3 actions, you can move up to 4 Range. You gain the flat-footed condition, and you can’t run if you must cross difficult terrain or can’t see where you’re going. Running provokes attacks of opportunity. You can run for a number of consecutive rounds equal to 3 + your Constitution Modifier. After this, you become Fatigued. 

All characters have a base Move speed of 1 Range per Move action. Certain Feats, Supernatural bonuses (like a Vampire's Alacrity), or having a Master or Legendary proficiency in Athletics might increase this speed or allow you to move additional Range increments. 

Move (1 Action) ⬤ ◌ ◌

You move up to 1 Range (or more, if you have a higher speed). Many nonstandard modes of movement are also covered under this action, including burrowing (using your natural burrow speed, if you have one), climbing and swimming (using either the Athletics skill or your natural climb or swim speed, if you have one). 

Behind 

You can declare your character Behind an ally. Any foe that tries to enter into Melee with you when you are Behind triggers an Attack of Opportunity from your ally. A weapon with the Reach property, such as a spear or polearm, does not trigger such an attack of opportunity. 

Step Behind (1 Action) ⬤ ◌ ◌

You declare yourself Behind an ally in Close Range. Any foe that tries to enter into Melee with you when you are Behind triggers an Attack of Opportunity from your ally. A weapon with the Reach property, such as a spear or polearm, does not trigger such an attack of opportunity. You remain Behind your ally until you are forcefully moved (for example, you are knocked prone, teleported, or bull-rushed) or combat ends.

Targeting 

Some effects require you to choose specific targets. You use these effects on creatures or objects as defined by the effect itself. You must have a line of effect to the target or targets of an effect you wish to use, and you must specifically choose the target or targets. If the effect takes a certain amount of time to activate (such as the casting time of a spell), you do not have to select your target or targets until you finish activating the effect.

Targeting can be difficult or impossible if your chosen creature is hidden from you, if the creature doesn’t match restrictions on who you can target, or if some other ability prevents it from being targeted, such as temporary immunity.

Some effects require a willing target. Only you can decide whether your PC is willing, and the GM decides whether an NPC is willing. Even if you or your character doesn’t know what the effect is, such as if your character is unconscious, you as the player still decide if you’re willing. You can declare yourself a willing target at any time as a free action, even if you're stunned or flat-footed; this merely requires you to inform another player and the GM that you are a willing target.

Some effects target or require an ally, or otherwise refer to an ally. This must be someone on your side, often another PC, but it might be a bystander you are trying to protect. You are not your own ally. If it isn’t clear, the GM decides who counts as an ally or an enemy.

A spell or area-style effect that targets multiple nearby enemies in a group can’t skip over enemies. You pick one target and attack the rest in order; you don’t skip all over the battlefield. Spells that say they target multiple nearby enemies but don’t specify that they have to be in a group are capable of sending magical energy in different directions, allowing spellcasters to choose targets from where they like. 

Some effects allow you to redirect the effect to new targets or areas after you activate it. Redirecting an effect is a single action that does not provoke attacks of opportunity.

Areas of Effect

Some effects occupy an area of a specified shape and size. An area effect always has a point of origin and extends out from that point. There are four types of areas: emanations, bursts, cones, and lines. Many area effects describe only the effects on creatures in the area. The GM determines any effects to the environment and unattended objects.

Burst

A burst covers an entire area covered in the description of the effect. A "close range" burst would affect all targets that would be in close range with a designated target or point of origin, be it a creature, object or terrain feature.

Cone

A cone shoots out from you in a direction specified by you. A cone only affects targets in a particular direction. So if you use a close-range cone on a target ahead of you, it won't affect those Behind you. Because it originates from your space and widens as it travels, it typically affects all targets situated in that general direction, including multiple targets in Melee with you if they are clustered together.

Emanation

An emanation expands from the target out to the described range. If the target changes their position or whereabouts in a fight, the emanation moves with them. For example, if you cast a cloak of flame on a foe, causing them to light their allies in Close range on fire, the target may choose to retreat away from their allies to protect them from the flames. Once the target is out close Close range of their allies, they would no longer be affected by the emanation. 

Line

A line shoots forth in a straight line, affecting all creatures in a line out to the Range indicated by the ability. If you target a creature with a line spell, it will also hit all targets Behind that target. 

Generally, a line can only affect one creature in Melee with you, but may affect more creatures further out. 

Line of Effect

When creating an effect, you usually need an unblocked path to the target of a spell, the origin point of an effect’s area, or the place where you create something with a spell or other ability. This is called a line of effect. You have a line of effect unless a creature is entirely behind a solid physical barrier. Visibility doesn’t matter for a line of effect, nor do chain link fences and other barriers that aren’t totally solid. However, transparent barriers, such as a plexiglass barrier, counteract lines of effect, even though you can see through them. If you’re unsure whether a barrier is solid enough, usually a 1-foot-square/30 cm square gap is enough to maintain a line of effect, though the GM makes the final call.

Line of Sight 

Some effects require you to have a line of sight to your target. As long as you can precisely sense the area (as described in Senses) and it is not blocked by a solid barrier, you have a line of sight. An area of darkness prevents line of sight if you don’t have darkvision, but chain-link fences and other obstacles that aren’t totally solid do not. If you’re unsure whether a barrier is solid enough to block line of sight, usually a 1-foot-square/30 cm square gap is enough to maintain a line of sight, though the GM makes the final call.