server-specific:
Everything related to any entity of the game, like player, chests, monster and items.
The server only sends information of a specific entity to the client when the player is close to that entity.
That also means we can't change position, or any other stats of those.
Any attempt of changing those will cause desync.
client-specific:
the map/minimap (map is generated on the server then given to the client), everything related to the player view, like fov, zoom, textures, lightning and particles.
Maybe a Skin-Hack is possible. Maybe not.