ACS (Action Coding Script?) is a compiled scripting language that works along side with the Hexen map format. The compiled scripts are stored in a map's BEHAVIOR lump in a binary format decoded by the zdoom/hexen engines. The license the ACS interpreter was released under is not GPL complaint, so there is no ACS support in Odamex. Here is an interestinf file I located which can aid in rewriting the interpreter. (formatting...)