bool IsClient()

Returns true if the script is running on a client.

Any running instance of ZDoom that has display and input capabilities is considered a client.

If the instance is a client, graphics effects (clientside actors and hud) should be enabled for it.

The following are clients:

• A singleplayer game.
• A client in client-server Zandronum multiplayer (but not the server).
• A client that is also acting as a server for other clients (this mode is not supported in Zandronum yet but other game engines have it).
• A player in (G)ZDoom multiplayer, which is not client-server, but makes all players run exactly the same game.

Using IsClient and IsServer correctly ensures that your mod runs correctly in both singleplayer and multiplayer:

if (IsServer())
{
    // Run serverside gameplay code.
}
if (IsClient())
{
    // Draw hud, spawn effects.
}