I think what robertlaird is saying is that if you're looking for "U" upgrade space (so you can buy a "value" fare and upgrade automatically as an MVPG or use your MVPG upgarde certs) you can use the low fare calendar to find dates when there is "A" space available and, because A generally equals U, you'll know what dates have available upgrade space (not sure if that's any clearer, but that's how I read it).