Accounts Module > Nominal Ledger > Utilities > Set up, second tab in Nominal Code
You CANNOT use the first hundred in the list as these are system coded ones, Scroll down the list to find "NOT USED".
You can change the type to 10/20/30.
10 = Sale/ 20 =Discount/30 = Cost of Sale
You can update this by selecting the chosen line, double-clicking to enter, and changing "Not Used" to your description,
Change type to 10/20/30 by ticking the box next to choice, and unticking the unwanted selection.
Do not touch the Linked Transaction Box.
Below is a video that details this further.