In World of Warships Blitz, shell types are types of ammo that will be utilize for dealing damage. There are three types of ammo in game: High-Explosive Shells, Armour-Piercing Shells, and Semi-Armour-Piercing Shells.

High-Explosive Shells are rounds that burst immediately upon hitting the target, dealing damage with a chance of setting fire. They have less penetration power than other rounds. 

When a ship is on fire, they deal 10% of the ship's maximum HP in 20 seconds. There can be up to 3 fires set on a single ship simultaneously dealing 30% of the ship's maximum HP in 20 seconds.

Two factors to reduce fire damage is the Commander's Skill, Extinguisher, which can reduce fire damage by 15%, totaling 8.5% of maximum HP per stack. You can also stop by using Damage Control Kit consumable. But do note that you have a limited amount of it so use it wisely! 

To reduce the chance of getting set on fire:

1. Increase Fire and Flooding Resistance through Equipment and Elite Bonuses

2. Purchasing the Deck Protection Modification I Equipment

3. Using the Commander Skill, Horizontal Protection Expert

4. Using Damage Control Kits with the Generalist Commander Skill

The best way to set fires is to target the superstructure or the deck of the ship. It has a higher chance of setting a fire.

Armour-Piercing Shells are standard rounds that deals high damage when they penetrate. They have more penetration than HE shells but have a chance to either ricochet (no damage) or overpenetrate (25% of shell damage), depending on the enemy ship's armour.

If AP rounds fully penetrates a ship's deck or belt, there is a chance to deal citadel damage. Citadel damage deals massive damage to the enemy ship. Citadel Damage increases, depending on the caliber. An average 152 mm caliber deals 1.5x more damage while 460 mm caliber from Yamato deals 3x more damage. You can find your Citadel Damage Rate under Guns.

Semi-Armour-Piercing Shells are unique rounds that deals very high damage in exchange of reducing penetration. They have less penetration power than AP shells but still more than HE shells. These shells are mostly unique to Italian ships and cannot over-penetrate.