Wind Load on Open Buildings

[mjs611]

Can anyone explain the difference between load cases A & B as shown in ASCE 7-05, figures 6-18 A,B,C,D? I can't find anywhere in the commentary or code that explains these two cases. Thanks!

 

[iv63]

My handwritten note says “max. variation in loading due to gusts at different times” (I do not remember where this note came from).
IV