From Exterminatus Manual
Veteran Marine (Vet)
Experienced 'Veteran' marines are often put in charge of combat squads of tactical marines. Over the decades of service they give to the chapter they earn the right to use more exotic equipment, often favouring more close combat weaponry and a leadership role. These 'Veterans' make up the second Imperial class, adding more equipment and speed to the Tactical class.
Veterans can be recognised by their white laurel markings on their helmets and skull motif knee pads.
The main battle weapon of Exterminatus Veteran marines is of a deadly combination of a Powersword and Plasma Pistol and Grenades. An alternative war gears set allos the veteran to swap his Plasma Pistol for a a more exotic Inferno Pistol, which does more damage, but has a much shorter effective range.
The Plasma Pistol fires relatively slow moving, but potent plasma bolts that are most effective at medium to short range. The Powersword can pack as big a wallop in close combat as anything else out there. Be carefull not to empty your plasma clip when you need to use your Powersword, since the Powersword cannot attack while the plasma pistol is being reloaded. Veterans are faster than other marines, use this to scout ahead of your team mates.
40K vs Exterminatus
In the tabletop game Veterans often have special abilities and are formed into their own squads, where as ours are more like sergants or squad leaders. Actually calling them squad leaders would undoubtedly have led players using this class as an excuse to boss about their team mates, which can be unpleasant for the other players, hence the less formal Veteran name.