NSF is a custom file format created by Naughty Dog for use in the Crash games. The root directory of a Crash game CD will have several sub-directories, with names S0, S1, S2, and S3; within these sub-directories are pairs of files with the name S00000**.NSD/.NSF. Each pair corresponds to a specific "level" in the game, and these files contain the game data for that level. An NSF file contains all of the resources used by the level currently being played.
It only serves as a container for chunks.