Probably means "Scan OUTs"
which means as you zone, and item that appears to be "out of stock" - you scan it with your zebra to see if any are in the back. Using the Zebra, you can create a fill batch that will pull those items out of the BR.
I usually scan OUTs and Critical Lows.
Critical Lows are items that don't have enough to cover all the "facings" - ie. item has 6 peg locations, but less than 6 are on the floor. Or item has 3 facings across on the shelf, but only 2 are on the floor.