The SAT format is used by the ACIS geometry modeling engine developed by Spatial Technologies⁵.  It directly represents the internal data structures of the ACIS modeller. That means if you load an SAT file into an ACIS-based CAD program, there's normally no translation at all: the file goes straight in. Hence this is not a "CAD Exchange" format as such: it is a CAD format.

You will most likely use the .SAT file format if you use an ACIS-based CAD program, and not use it at all if you don't. Again, this format is a smooth, continuous object representation. Usage is exactly as per the IGES file format.