Damage type
From DoomRL Wiki
Every damage source has a damage type associated with it. There are five types of damage: physical, shrapnel, fire, plasma, and acid. Each is, of course, different from the others in some way, though some are more common than others.
Physical Damage
For every point of armor the target has, they take one less damage per hit. Physical damage cannot destroy walls. It is dealt by way of many melee attacks and by most weapons that use 10mm ammo.
Shrapnel Damage
For every point of armor the target has, they take two less damage per hit (although this goes to zero for the player if they have the Army of the Dead master trait). Shrapnel damage cannot destroy walls. It is limited to most shotguns.
Fire Damage
Fire damage treats protection normally (one less per point), but can destroy walls. Additionally, fire damage is the best for gibbing enemies. Despite the similar names, there is no connection between fire damage and Fireangel.
Plasma Damage
Plasma Damage halves armor protection, rounded down. Gibbing is easier. Most weapons that use power cells do plasma damage. Additionally, several enemy projectiles cause plasma (even some you may not expect).
Acid Damage
Acid damage damages armor durability twice as much as other damage types. Gibbing is easier.