There are 4 different zones on the server: green, blue, conflict and red. Every zone has its own rules you should adhere to.
Your main objective in the green zone is to patrol the airspace, ensure that civilian aircraft stay out of this zone, and spot suspects for ground units.
It is prohibited to land on the main roads and in the square. Landing in the Green Zone outside of these places is only allowed during active situations that started outside the Green Zone such as following a truck or a suspect fleeing into the zone. Beware that you refrain from deafening civilians by flying too low.
A Blue Zone is KOS for Police against rebels. There are no special rules to be followed inside these zones.
A Conflict Zone is temporary. You may enter the Conflict Zone with your helicopter but should maintain a high altitude to make clear that you are not a participant in the Conflict Zone.
A Red zone is a no-go zone for every cop and therefore you are not allowed to fly inside of these zones.
However keep in mind that, whenever you are chasing someone and he flees into this zone, you are allowed to follow him inside.