Custom emoji

A creator with memberships enabled uploads small square images, gives each one a shortcode, and assigns them to membership levels. A member's set grows with how long they have been paying, or with the tier they sit on. In practice they work as a visible badge of belonging, since a regular typing a channel's in-joke emoji is signalling membership to everybody else in chat. They only work inside that channel, so they carry no value anywhere else on YouTube. YouTube reviews uploaded emoji before they go live.