Update: a quick AI query says this:
How Preset Recall Works on FBV3 and Spider V
Bank Up / Down Arrows: Scroll up and down through the 32 available user and factory banks. [1, 2]
Footswitches A, B, C, and D: Directly recall the four specific presets (A, B, C, or D) contained within the currently selected bank. [1, 2]
Second Row Footswitches (FS1–FS5): Control effect bypass and individual parameters within the active preset rather than changing entire amplifier presets. FS1 and FS5 are locked to compressor and reverb, while FS2, FS3, and FS4 control active preset effects (such as modulation, delay, or drive). [1, 2]