Be careful! It seems that, partway in development, they switched things up. Each level has another archive that goes with it, the act_lz7 archive (the suffix means the archive itself is lz77 compressed) that contains level-specific actor collection files (where actor, in this case, means objects like enemies, items, gimmicks, etc), and there is also the archive act_com_lz7.narc which contains generic stuff like special effects and HUD elements. The ones in the /act directory seem to be residual files that are left over from a previous state of development (in fact they still lingered in the E3 beta but I don't think the game actually uses them even then!) and was cleaned up and cleaned out in Rush Adventure and Colors DS. The actor collection files themselves contain graphic placement data, animation data, palettes, and graphics for certain object(s). For example, the rocket launcher Egg Pawn's file contains the badnik itself, and the rockets it fires.