'A' and 'B' Guest Lists

I've never heard of them until I started using LIweddings. What is the reasoning for these?

I thought you would just have everyone's name on one list that you want to invite and that's that.....

Oh, the things you learn! lol lol lol

my impression is that for those couples on a budget, the A list is for those that you want there no matter what- the b list would be for those that you would have if you could. so those on the a list who rsvp that they can't make it open up a spot for those on the b list.

I hope this made sense.