Sale Type Codes (S-Codes) are used to indicate whether a product is a regular sale (the most common), return, backordered item, consigned item, etc.  There are about a dozen codes.  If you forget the appropriate code letter, you can bring up a reminder list.  More information: List of Sale Type Codes.

To bring up the reminder list, select EDIT or PRODUCT TOOLS and PASTE FROM ITEM S-CODE from the Menu Bar on top of the screen.  You can also access the list by pressing Ctrl+I or clicking the right mouse button and choosing it from the Speed Menu.

The "List of Sales Codes" looks like:

You must have the cursor on a line in the Item Area (the place where product quantities, catalog numbers and products are entered).  If the cursor is not in the Item Area, the S-Code list will not appear.

Once the list comes up, you can select the appropriate Sale Type code.  Press OK (or ENTER on the keyboard) and the code letter will be pasted in the S-Column in the item row where your cursor is located.