.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. When Halo 2 Vista was initially released, an ".ass error" would occur, showing a picture of Bungie employee Charlie "Chucky" Goughs uncovered backside. This has since been patched.
If you pay attention to the picture, it has 7 warnings, a reference to Bungie's favorite number.