The Payload Fairing encloses and protects the spacecraft
during ground  operations and launch vehicle ascent.

Two PLF lengths are available at 13305 mm and the standard 15255 mm. Although of similar design, there are specific useable volumes for the two fairing types tailored to individual adapter systems. Specific adapters take into account required adapter clearances for installation and required flight clearances with the adapter structure.

Please see Section 4 of the Proton Mission Planner’s Guide for further detail