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 controls a player on 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.

• bool IsServer() – returns true if the script is running on a server.
• bool IsClient() – returns true if the script is running on a client.

• bool IsServerOnly() – returns true if the script is running on a server that is not also client.
• bool IsClientOnly() – returns true if the script is running on a client that is not also server.

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