SARJ is an archiving utility for DOS. It was developed by Frederic Taes and TSF SoftWare, and distributed as freeware. The name is probably derived from "Solid archives with ARJ", or something like that.
SARJ produces two uncompressed files, with extensions .DIR and .SLD, and puts them in a compressed archive, using an external ARJ utility. The theory is that a single large file usually compresses better than multiple small files.
The .DIR file is a text-based file formatted like a directory listing. The .SLD file is apparently just a raw concatenation of the files in the archive.
SARJ has some sort of support for archive formats other than ARJ/SRJ, but more research is needed to understand the nature of that support.
The documentation says that in SARJ v1.12, the default filename extension of its ARJ-based files was changed to .SRJ. It doesn't say what it was changed from (maybe just .ARJ?).