Plasma is a non-standard damage type used exclusively by humans: both by Templar and Cultists. No known Xeno or Narvidian uses Plasma in its attacks. Plasma is not currently an environmental hazard on any missions.
Like all non-standard damage types, rolls for dealing plasma damage range between 30% and 100% of the stated damage. This differs from bonus damage from other sources which is from 0% to 100% of the stated damage. This, combined with the fact that non-standard damage types ignore armor makes it a powerful method of dealing with most foes. (Non-standard attacks do have to contend with opposing Resistance and Res, but those are usually very weak defenses.)
Known Plasma Users
Plasma damage is an optional damage type for: a Soldier (range), Paladin (melee), Berserker (melee), Captain (both), and Scout (both, but only through the use of the Wraithbone Kit [RELIC] shared with the Captain). Its presence is dependent on which weapon has been selected, as it is not a damage type present on every weapon available. The only enemy to use it are the Heavy Troopers, who are an uncommon spawn.