A courier is a BTSC message sent legally from the Messenger to another player (often sent to the host and relayed to the intended recipient). The recipient of a courier is usually chosen by player name, but variations exist that allow players to courier a specific role (see: Picard in Mafia All Stars).
Although typically an action carried out at night, a courier sequence was introduced to sequence voting by Araver during Amber Mafia. Because of the large volume of couriers in Amber, direct messaging from one player to another was allowed, but isn't typically the standard. Confirmation should always go through the current host.