Passage: John travels to The Continental Hotel in New York City. Winston, the owner of the hotel, reminds John that if he rejects the marker, he will be violating one of the two unbreakable rules of the underworld: no blood on Continental grounds, and every marker must be honored. John reluctantly accepts his commitment and meets with Santino, who tasks him with assassinating his sister Gianna so he can claim her seat on the "High Table", a council of high-level crime lords. Santino sends Ares, his mute personal bodyguard, to surveil John.

Question: Aside from not spilling blood on the Continental grounds what else is considered an unbreakable rule of the underworld?
every marker must be honored