BSA -
Bzip2
Solid
Archive. Its a very simple linear file archive format for packing multiple files into a solid BZIP2 stream. While partial decompression is possible, you have to decompress the stream before that file to reach and decompress said file, since its a solid archive. It is best suited for situations where you want to decompress the entire archive at once. This is essentially a custom replacement for tar.bz2, lighter and easier to understand and implement.