Doesn't mean a thing. There's no reason for you to ever need both. (To be clear: For all we know, everyone gets both numbers now. But you only ever need to know one.) The B file number is what they typically give you. You won't need the F file number for anything if you already have the B number. It's NOT standard (or at least, didn't used to be) for them to give people F numbers. They seem to have started doing it more recently, but perhaps that is tied more into their ways of dealing with the backlog. Example: give all new files a preliminary F file number first, B later? It seems like, lately, the people getting the F numbers first are the ones who've asked Buffalo for file numbers early in the process, before Buffalo even gives them an official AOR with the B number.
I could be wrong about the numbers having to do with backlog or asking for file numbers before they're ready to give you one, but that's what I've gathered from the posts I've seen. It could also just be a product of their new computer system, i.e. perhaps more electronic filing now.
