"PM_UPD_*" files are created when you update your PlotMaker layouts from a Teamwork (plp) file. In such case PlotMaker locks the PLP for a little time while it duplicates the plp (this is the "PM_UPD_*") then releases the original plp so that other users can send and recieve, and background ArchiCAD works from the "PM_UPD_*" file. When you exit PlotMaker, the "PM_UPD_*" is deleted.
This change in ArchiCAD 9 solves some issues that happened when teammates were sending changes to the plp while others were updating views from the same plp.
Gergely Kmethy
VP, Customer Success, Graphisoft