Break Siege and arriving armies

[GrudgeBringer]

I never played SP and in MP it doesn't come into play that many times so I thought I would ask.

If you are going to break a siege and have armies arriving from an adjacent province, does it become 2 battles which for the most part would render the rienforcements useless.

OR....because you ARE using break siege, you are ordering your troops in the fort to come out and fight also, all troops are used.

I could understand if you didn't use break siege that it would be a battle of troops showing up and the troops inside the fort do not get involved....sometimes common sense (lol, at least what I think common sense is)doesn't seem to apply.

Thanks