An embargo is when characters are blocked from registering.
Already registered[]
Every character can only be registered once. Even in later tourneys, you can't register old characters. However, special event losers aren't registered, so if Miles "Tails" Prower loses the Sonic Event, he can be registered later.
Third representative[]
Each universe has a limit of only two characters per tourney. As Harry Potter and Severus Snape has been registered in Tourney 1, no more Harry Potter characters can be registered in Tourney 2.
Event embargo[]
During a special event, no characters from that universe are allowed in the main tourney until the event is over.