I can't emphasize the importance of nymph emergence during mayfly hatches and how intensely trout key on the stage. All of my literature review and observations at Livingston's spring creek support this theory. We are not talking about "misc nymph fly patterns fished under bobber". Rather, mayfly nymph as creature. Indeed fishing for this stage is as technical as dry-fly fishing for duns.
FA stands for Floating & Ascending, meant to be fished in the film or slightly below during the hatch. Situation goes: trout are obviously rising but without breaking the surface. The matching dry-fly dun and its best presentation won't do a job. That's because trout are keying on ascending nymphs in the water column rather than making effort to feed on duns on the surface or in the air. That's when floating and ascending nymph patterns become key-players.
Fishing report: http://leftyangler.blogspot.com
Fly catalogue: http://leftytyer.blogspot.com