SITX is the extended format of the archive format SIT. SITX or SIT is a commercial format of the software Stuffit for archiving and compressing data. The format originally came from the Apple Macintosh and was also the long-standing archival format on this platform. Windows versions of Stuffit have been available since 2007, but so far SITX has no meaning there.
To create a Sitx file you need at least StuffIt, the Deluxe version is not necessary. In addition to high encryption (eg RC4 512Bit, Blowfish 448Bit) the SITX format offers above all special recognition routines for various data types. For example, jpegs, pdf files or MP3s are compressed according to the manufacturer's statement with a respective optimized method. To make SITX files usable for older versions of Stuffit, you can enable a compatibility option.
For unpacking a SITX file, you can also fall back on the free of charge provided by the manufacturer Smith Micro Stuffit Expander. On the Apple platform included the Stuffit Expander up to OSX 10.3, from OSX 10.4 you have to download it yourself from the manufacturer side.