What does "FB" mean? I can only guess it's some kind of protective device because of F, like with FU for fuse.
Is there any comprehensive online guide for such kind of symbols?
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From your description, sounds like an abbreviation for Ferrite Bead.
FB is the designator for a ferrite bead. A FB acts a lot like an inductor, in that it filters out AC, and they slow transition of DC signals.
Best list of designators I've run across is in Wikipedia.