RndPostProc

RndPostProc (short for render post-processing) is an asset used to define post processing filters that appear over the venue during gameplay. Introduced in Guitar Hero 2, Harmonix began heavily using them in the Rock Band games to enhance the visual experience. Songs can call upon a specific post processing effect via an event in the MIDI, or via the song.anim file inside the song's Milo scene using a tool such as MiloMod.

In The Beatles: Rock Band and later, it is possible to bundle a custom RndPostProc inside a song's Milo scene and then reference it via the song.anim for creating custom post processing effects without modifying the game itself.

Harmonix's description for this is:
A PostProc drives post-processing effects.