PWM - is the extension for the WM Keeper WinPro (Classic) purse file in which the operations history of your WMID and WM-chat is kept.
The extension (file name extension, in full) is a sequence of symbols, separated from the base filename by a dot, applied to indicate the file format. Don't mix up PWM and KWM extensions. KWM is the extension for key files.

See also:
Purse file
WMC purse history