The Not Discountable field controls whether Clerks (with insufficient security) may change the item's price on Sales Transactions.
The Security Level "Edit Line Item Price of Non Discountable" allows Clerks with an adequate Security Level to override the Not Discountable setting on the More Data Inventory Screen.
When enabled, a Clerk with insufficient Security is prevented from using the [Tab] key to navigate to the Price Field when the item is entered on a Sales Transaction.
When disabled, any Clerk is able to alter the line item price (as long as the Clerk's Security Level also meets the Security Level set for Edit Line Item Price).
Note: This setting affects only the ability to edit the line item Price field. It does not affect discounts entered in the Discount field on the Complete Sale Screen, nor does it affect discounts resulting from the use of Price Tables.
See:
Inventory Database - More Data Screen Fields