Permission to board

From YPPedia

Some crews require players to ask permission to board (often abbreviated PTB) before they board a ship. This may be considered a formality, in which case permission will be automatically granted. In other cases, there may be reasons why others might not be welcome on a crew ship. For example, a pillage might be private, or devoted to training a new officer.

People who are making trade runs instead of pillaging, or are practicing navigation to memorize routes may want to avoid sea battles or not wish to pay any extra hands. Having additional players on board can also cause more and/or harder brigands to spawn, threatening the cargo. The trader may also prefer having a bot to a player simply because they are easier to order around.

Ships may also be involved in events and be limited by the event rules as to how many people may be aboard at a time.