The On Sale flag is used to identify items which should default to Sale Price rather than Base Price.
When an item marked On Sale is entered on s Sales Transaction, Sales Price is used instead of Base Price. However, if a lower price is available through a valid Price Table, that price is used instead of the Sale Price.
Note: Promotional pricing is better accomplished, with greater flexibility and control, through the use of Price Tables.
See:
Inventory Database - Main Screen Fields