The smaller the micron number the finer the bag. We tend to use the 200micron bag during fall to get most of the debris in the pool. We find that the 200 is perfect because is doesn’t clog as much as the 100micron. Summertime we do use mostly 100micron bag along with system vacuuming with a hose when needed. This 400micron bag came in clutch for the dirt/mulch that got inside the spa!