The craft can be distinguished from those following by their bent solar panels and their use of the Igla automatic docking navigation system, which required special radar antennas.

The 7K-OK was primarily intended as a variant of the 7K-LOK (the lunar mission Soyuz) for Earth orbital testing.

The early Soyuz models also sported an external toroidal fuel tank surrounding the engines and meant to store extra propellant for lunar flights, but it was left empty on the first nine flights.

After the spacecraft's purpose was changed to space station ferry duties, the tank was removed.

The main modification was the addition of the new SSVP docking system that allowed internal crew transfer, which was performed for the first time on the Salyut 1 space station by Soyuz 11.