Hmm..looks to be just the maximum variation in loading due to gusts at different times. It says to check all load cases, so I assume you check Case A and Case B for each situation. I don't see any text reference to indicate the letters actually correspond to a specific design condition, and don't see anything to indicate you'd use Case A this time and Case B for some other situation.
I think they are the lower and upper bounds for all possible wind flows or pressures over a pitched roof. This is similar for a house with a gable roof. Try calling ASCE to get the explanation. Good Luck.
