BZ2
BZ2 (bzip2) is a single-file compression format, which is slightly slower but slightly more efficient than ZIP's standard 
Deflate compression.
It is optionally used by 
Source 1 FastDL servers to reduce network bandwidth usage, by sending the compressed file, which is then decompressed by the client. This does not reduce storage usage, however; while the FastDL server only needs the BZ2 version, the server needs both the decompressed and BZ2 version, and the client will only keep the decompressed version.
