This protocol looks at the file and eliminates all the empty spaces in the file. Also if you get information on one of these files from within ResEdit, you will see these names under File Type. The programs that open these documents are all shareware and are available from BBSes or other Mac users. SIT means Stuffit format. Stuffit doesn't seem to be as popular as Compact Pro, also I've noticed that there seem to be multiple versions of it in use and sometimes one version of Stuffit won't open a particular Stuffit archive.
PIT means Packit format.
This is an older format no longer in use. If you run into one of these, UnStuffit will handle it. CPT means Compact Pro format. Compact Pro appears to be the leading archival format in the Mac world. The file is compressed along with the information needed to decompress it.