Thank you for the info but it shouldn't matter how I think of them so I think that comment was unnecessary. They don't physically store stuff but they do store all your info including fast pass reservation so that when you scan it the reader knows you have a pass.
Actually, they are like a 'liason' between the scanner and your account. Which is why you could take mulitple MBs and use them all on the same trip!...as long as they are in your name anyway.
Anyway...to answer your question....when it comes time to make your FP+ choices, you will go to your MDX account. You will bring up each day of your stay, and choose the people that you want to make FP+ choices for...I imagine that it will be for your entire family. Then, a screen will come up, giving you the possible attractions with possible return times. You make your first choice. Then do the same thing again for a second choice and a third choice. And then do it for each day of your stay. Then, when you go into your reservations page, on MDX, you will see the FP+.
But...everyone in your family has to be linked to you in order for them to get included in your choices. Otherwise, they will have to make their own choices. They will have to have their own MDX accounts that are linked to yours. It's all easier than it sounds!!!
Then, when it's time to go to that particular attraction and use your FP+, each person will scan their band on the scanner at the entry point for the attraction. And that's how the CM knows you all have a FP+ that is valid.
Hope this helps a bit...it can be confusing until you actually use it.