Jokers generally can't be used as a single tile or as part of a pair, and they're barred entirely from hands marked Jokerless on the card.
Because a lone Joker or a Joker-pair would be too easy to fake, the rules restrict them to sets of three or more matching tiles. Some card categories — most notably Singles and Pairs hands — forbid Jokers altogether.
Example
You can't use a Joker to stand in for the single tile required on some hands, or as one half of a required pair.
