From Exterminatus Manual
Specialist Marine (Special)
Most tactical squads contain one marine who is trained in the use of support weapons, which are often rare, difficult to use or otherwise specialised in their function. The 'Specialist' class in Exterminatus is a Tactical marine who has traded in his Boltgun (general purpose combat rifle) for one of these weapons.
Specialists can be recognised by their distinctive beaked helmets that hark back to the old MK6 power armor.
Meltaguns have very limited range, but do more damage close up, good for ambushing bigger tyranids, they can slice up even a Canrifex at close range.
Flamers have greater range and area of effect, but are weaker than meltaguns.
Plasmguns have longer range and hititng power, but fire slowly can have few shots between reloads.
Specilists are effective counters to the Tyranid lictor and carnifex classes, which still remaining effective fighters against other classes. The main weapons of this class are very specilized in their uses, sometimes it will be more appropriate to use a boltpistol, particulalrly against Genestealers.