The Mark-down setting is used to discount the Price by the Percentage (or Amount) entered in the Mark-Down amount field. Use the Percent/Dollar button to toggle between Percentage and Amount.
The Mark-down is typically taken from one of the Price fields (selected by the From setting). Mark-down may be changed to Mark-up or Profit by pressing the [F9] key from the Mark-down amount field.
The Mark-up option is typically used to mark-up from a Cost field. For example, 20% above Last Cost (in a Price Table used for Employees, or for resellers).
See:
Price Tables - General Information
Configuration Manager Contents