A SITX file is a compressed file saved in the Stuffit X archive format, the default format used by StuffIt 7.0 and later. It features Smith Micro's exclusive ATOM compression technology and offers up to 20% more compression than standard Stuffit (.SIT) archives.

The SITX format supports long filenames, preservation of UNIX (i.e. OS X) and Windows file permissions, and error correction, which fixes data transmission errors. It also supports password protection using 512-bit RC4 encryption or other encryption methods, including AES, DES, and Blowfish. Version 9 and later provides 15-30% compression of JPEG images. NOTE: StuffIt was previously developed by Allume Systems (formerly Aladdin Systems), but is now developed and distributed by Smith Micro.