Can you explain the difference between a prospective Upper Payment Limit (UPL) and a retrospective UPL?

The difference between a prospective and retrospective UPL is in the relationship between the UPL demonstration period and the date when the UPL is submitted. For a UPL demonstration period of 7/1/2018 to 6/30/2019, a UPL is considered retrospective when it is submitted on or after the start of the demonstration period (on or after 7/1/2018). Using the same UPL demonstration period (7/1/2018 to 6/30/2019), a UPL is considered prospective if it is submitted prior to 7/1/2018.