I believe it stands for alcohol. It will show up if you enter a food from a recipe you made in which you listed a flavor extract ... vanilla, for example ... as an ingredient. Of course, it shows up if you drink alcohol, too, or if you list any other food that contains alcohol.

If you click on Monthly View in the menu on the left of your My P.L.A.N. page, then scroll down to the bottom, you'll find a Legend that lists all the letters and what they stand for.