When you open Shopkeeper after some time, you will see how many new sales came in while you were away.
For example, you get 6 new sales:

The little green numbers will show you exactly which products are those 6 new sales:

There is a timer of about one minute, after which the little green numbers disappear, together with the green info notification on top.
We consider those new sales "Viewed".
So the little green numbers help you to quickly see what new units just recently sold.