broadcasting roles

I poked around but couldn’t figure it out nor find docs about how to broadcast ROLES

in Broadcast > Settings > Roles it shows roles but not too clear how this works. if I could create a role on my primary site, assign a user to that role, and then broadcast the role to a specific subsite or subsites (not all) – and then when the user visits that subsite they have role privileges — it would be great, but I don’t see how that works

THANKS