I've got to plead ignorance on this, but what does "bumped due to weight and balance considerations" mean at AA?

I know on the RJ, you can get into a situation where the JS can't be occupied. You need a lot of ballast to get back in CG, but when you try that, you will be overweight.

I'm assuming that in the context you guys are referring to, "weight and balance considerations" means something else. Could someone enlighten me?