SBStudio is a tracker for MS-DOS. It produces modules with the extension .pac, which consist of a song file (.son) with the necessary sound files (.sou) attached.
There are two versions of the PAC file format. v1.4 .pac modules are produced by SBStudio 2 and v1.6 modules by SBStudio 3. The second link in the information section describes the differences between the two versions.