There is only one file type that uses the .USM extension:
USM Game Video File
Video file used by various video games, often console games; similar to Bink video files (.BIK extension) and commonly stores cutscene video that is played back during gameplay; sometimes converted to other video formats by VGM collectors.
NOTE: Some USM files can be demuxed with VGMToolbox and deinterleaved with the usm_deinterleave program included with the utf_tab suite.
Updated: December 3, 2014