Tokens are used throughout the game to record or remind you of certain game effects. When instructed to do so, tokens can be placed in any space on the board, on character sheets, or on Inventions or Items.
Note that there is a difference between tokens and markers. Tokens are cardboard, while markers are wooden. Different rules apply to markers.
No space on the board can have more than one of the same token (except Special Wounds, which can be duplicated). If you are supposed to add a token to a space, but you can’t because there is already an identical token there, there is no penalty and this does not count as unfulfilled demand.