"Incremental Sales are purchases made by customers in addition to what
they intended to buy when they entered your store. The tourist who
goes to a gift shop in a resort town looking for a trinket, but buys
an expensive bowl instead, is one example. The museum visitor, who
wants a book about the museum's exhibits, but also ends up buying a
gift for their child's teacher, is another example. The hospital staff
person, who intended to spend $25 on a gift for Aunt Sarah's birthday,
but spends $50 including two additional items, is a third example."
This extract is taken from a site that goes into great detail about
other things that Incremental Sales that you might find interesting.
Hope this Helps.