in multiplayer games its easier to give more trust to the clients. Like in fps games you can generally take the position of a player as valid without having to check it on the server. Stuff like that uses movement prediction to show the location of the players to the other players.
That allows the actual position of the player to be updated only when that player stops or starts moving. Everything else is guessed by the other clients.
MMOs are kinda different. Where in fps games almost all the info can come from the client and generally be trusted in an mmo almost all the data is server side and mirrored only whats required to the clients.
Then the mmo client uses predictive movement for the mobs and other players based on the initial position it got from the server.
Since almost all data is server side in mmos you dont trust data from the clients. Basically you check the data received from the client to make sure its valid before updating the server values and giving those values to the connected clients.
Thats also generally why you see less lag in mmos and they can be played with a slower connection than online fps games. All the data crunching is server side and the basic info is then reflected to the clients only when they need it
General info but hopefully itll help out a lil