From Exterminatus Manual
Tactical Marine (Tactical)
The majority of the 2nd company's marines are formed into flexible 'Tactical' squads. The majority of marines in a Tactical squad are equipped to deal with all sort of situations, these make up the Tactical class in Exterminatus and are the basic fighting class of the Imperial team.
Tactical marines get a choice of two selections of waregear. The first option is the awesomely powerful Boltgun, a few Grenades, and a Knife. This is well suited to medium range combat and can switch to a knife for extremely close conditions. The second option swaps his seperate boltgun and knife for a combination of Bolt Pistol and Chainsword, along with a larger compliment of Grenades. This combination allows the marine to react much faster in close quaters and gives him a better chance at fighting back when attacked by xenos with melee attacks.
The Boltgun is a slow cycling weapon, take your time, aim, make your shots count. If you empty the bolt pistol's clip it will not reload automatically in case you need to use your chainsword to defend yourself.
40K vs Exterminatus
The war gear set featuring the Chainsword and Boltpistol is called the "Grenadier", a term you wont find in any space marine codex, but clearly differentiates the sets.