The SP command is used to specify the price of a product at the time of scanning.

This command supports two syntaxes.  The first: '[price]SP[SKU][enter]' where '[price]' is the 'special price' for the product with SKU '[SKU]'. The price will be multiplied by 0.01 as the cash command does.  Highlighting an existing product and omitting SKU will allow you to override the price on already-added items.

The second syntax allows you specify a percentage off: '[percent-off]%SP[SKU][enter]' where 'percent-off' is the discount to apply to the sell price of the item (e.g. '50' for 50%).
 
If you are using the touch screen interface you should enter the price, click 'SP' and scan the item.