i think it sets how long the meter stay alive (as catchlights said above) and the meter info stays up on the small top LCD and in the viewfinder after you take your finger off the shutter button. You can test this by half press the shuttle button but don't shoot, and verify this. If you wanna save power, set it to 3sec. If you work very slow and don't mind saving the batteries, then set it to 30sec. The default is 10sec. I set mine to 30sec...