In SA-MP, you can place a player in one of over different virtual worlds.

: Moving a player to a private world for questioning or testing. For Server Owners: Implementation

: Creating separate instances of the same race track for different pairs of players.

If you are looking to "download" this functionality for your server, you simply need to use the following already included in the SA-MP Wiki :

: Ensures vehicles are only visible to players in the same world. For Players: Can I download this?

: Multiple players can enter the same interior "house" but be in different virtual worlds so they don't see each other.