Think of it this way, in the advanced rules you have a limited number of actions so in the planning phase you decide to plan a boarding action and so place the crossed swords counter in one of the action box.

Once planning has been done you reveal the crew actions, this would show you have a boarding party prepared.

When it comes to the boarding phase, after the gunnery and musketry phase you can then declare the blank counter, so although you prepared a boarding party you are not using it.
Although I have no idea why you would prepare a boarding party and then not use it on defence if you were being boarded.
On offence I can see you could choose not to go over snd wait to use cannister on the crew in the next turn before sending over.

The crux being you must plan a boarding party in the planning phase but just because you planned it you dont have to use it