Hmmm.. I wouldn't think so.. What would be the point of having a "Robitussin FLU" then? I always thought the 2-letter intials had to do with the active ingredients in the syrup.. Robitussin DM has Dextromethorphan.. Robitussin PE has Pseudoephedrine.. I get stuck at CF though.. ahahah.. Guiafenesin has no C's.. and neither do the other two ingredients..
I always thoguht it was for "cough" or something. It's not any sort of official abbreviation. If you really want to know I guess you could call the consumer product information number on the bottle if there is one.