It can be used for viewing and editing-and occasionally fixing-various things about the Sims themselves, though it can also work with objects, clothes, and meshes. It is compatible with The Sims 2, as well as all of its expansion packs and The Sims Stories series. SimPE is a third-party program which allows players to view (and often work with) the 'package' files that The Sims 2 uses.
