What exactly needs to be replicated in a multiplayer game?

I’m new to multiplayer development, and if I’m making a chess-like, 1v1, multiplayer RTS game, what do I need to replicate? I have a base character class for the players to possess, a parent piece class which all of the different “pieces” derive from, and a “tile” actor which the “board” is made out of. Do I need to “replicate” all of these? Do I need to replicate all of their variables?

Thanks in advance