You can check if a script is running serverside, clientside or in singleplayer using functions provided by ACSUtils.

• Server – an engine instance that runs the gameplay code.
• Client – an engine instance that runs the rendering code and provides input to the server.

In singleplayer, the same engine instance is both a server and a client. Both IsServer() and IsClient() return true in singleplayer.

In ZDoom and GZDoom multiplayer (but not Zandronum's), all players are both servers and clients, just like in singleplayer.

The definitions above enable writing ACS code that functions correctly in both singleplayer and multiplayer without singleplayer being a special case.