The game uses a dedicated client for gameplay which forces client-server setup even for local offline singleplayer. The server is likely automatically started by the client in the background with no separate action from the player.
Alternate name: Client-server model
The first video game about Dedicated client was released on June 22, 1996.
More modern take on this is that the clients also maintain some form of understanding of the game logic (mainly things indirectly related to graphics and sound) to allow fairly good network prediction code, hiding away some latency issues (making lag less visible).