Sale Mode is a store-wide or per-dealer discount your store turns on — for example, 20% off your items.
You can keep a single item out of Sale Mode with the exclude from sale flag. That item prints a FIRM badge on its tag and won't be discounted.
Sale Mode and a manual line discount don't stack — whichever is larger wins. Only your store admin can set the Sale Mode percentage; you control the per-item exclude flag.