.ass or amalgam scene specification is a source file type. An intermediary format file produced by the 3ds Max exporter. When run through Halo 2 Tool, .ass files generate .scenario and .scenario_structure_bsp files, which can then be edited by other tools. The filetype is used within Bungie's Halo games...
Bungie in the 90s and early-00s must have been a weird place to work. I remember their blog posts from that era.