Use this format when you are compressing files to save disk space.
ΓÇó File type, creator and flags are preserved; compression followed by decompression gives you an identical file to the original.
ΓÇó Both the data fork and resource fork are compressed. In the worst case, the file can grow by 48 bytes; forks are copied if they cannot be compressed, but the header is always used. But you don't need to worry how it's done: it works.
ΓÇó This format is compatible with Unix "compress" provided the header is stripped from the file, and the file is split into two files representing the resource and data forks. If anyone has a need for this, I can supply details.