Conditions: is blank / is not blank
The following conditions:
will allow you to use individual mapping methods for individual products or product groups.
You can assign some values to products depending on what whether a given value is blank or not.
You can use this option to rename sale_price, if the value of sale_price field is not blank. Otherwise, as you can see below, the value of price field will be used.
You can of course combine this one with many other conditions.