What is an Invoice Concession?

 We recently upgraded to Projects Fam Pack M and it introduced a new option under the "Credit" button on the Invoice Review Screen.   Theres Cancel, Write Off, and now Concession.

What is it?  I know it creates a Credit Memo with the invoice class = "Concession" but when would I use that?  It seem to act exactly like a Write Off.  I guess the advantage I see is that you can apply a concession at the invoice line level.  However, would I be able to Cancel the other lines and rebill transactions from the other AR lines?

Thanks in advance.