The MP5SD is an MP5 with the addition of an integral suppressor, to reduce sound and muzzle flash. Still using the 9mm round. Best used if silence is to your advantage.
The MP5SD has a lighter bolt, as opposed to the other variants. This is for balance, as the suppressor is made of aluminum, and is welded to the barrel.
Like any other MP5s in the series, it uses the 9mm pistol round.