This module is customarily mated to the Soyuz Crew Capsule, and is seen docked with the MSS in Mission 11 and Mission 19. It will be unlocked in Mission 20. Then, it is used in conjunction with the N1-LOK-LK rocket in Mission 21.
There are two different designs of this module. The standard gray-colored variant is accessable to game players for regular use; it is not shown being equipped with communications antennas. The other variant is dark green in color and has two small communications antennas attached to its hull. This variant is found docked to the MSS space station during Career Mode or after Mission 11 in sandbox (after mission 11 there are 2 docked, after Mission 19 there is only 1).
Why the green Soyuz Docking Module is exclusive to the MSS is currently unknown.