It is a special kind of fairing that is used in the N1-LK-LOK rocket, and it houses the LK lander while acting as a connector between the N1 Block V and the LOK spacecraft. It is taller than the Saturn V Payload Fairing, and can mount a small payload directly without the use of a tug.
when you want to carry 2 medium-large sized payloads.
It can also be used for multiple payloads on a single rocket.
Note: When carrying a tall payload in the N1's fairing, or the Saturn V's fairing, usually you can only fit an LOK.