There is only one file type that uses the .XAS extension:
Pro/ENGINEER Assembly Instance Accelerator File
File created by Pro/ENGINEER, a program used to design manufactured components and processes; contains an instance of an .ASM assembly file, which is a particular parametric definition from a "family table" of assemblies within the ASM file.
XAS files reduce the amount of time required to retrieve specific assembly instances since Pro/ENGINEER does not have to load the "generic" geometry and all "family table" information first.
Updated: August 22, 2010