An XPSX file is the file format used by xpdrainage for surfaces created within the software. it contains both the raw data (vertices and breaklines) as well as the fully triangulated surface.
When data is loaded form another file source (.txt, .csv, .xls, .xlsx, .xml, etc), or entered directly by the user, the software will build a triangulated surface from this information. This is then stored to an external file to allow it to be reused in difference phases or designs as required. The file will be named automatically based on the current file name if present, otherwise a filename must be entered.