There are two different (incompatible but very similar) varieties of these files, a PC format and a Mac format. These files contains one instrument definition (per file) and references a bunch of external waveform files. For the PC format, the external files is .WAV files. For the Mac format (usually .INS) they can be either .AIF(F) or .SD2(F) files. When reading an .INS file you should put all these external files into the same directory as the .INS file. It is very important that these external files retain the original filename that they were written - so when transferring Mac files to a PC, take extra care to retain the 'long' file names. And in case of referenced .SD2 files you must transfer it to a 'flattened' MacBinary format file.