I think what you're asking is do the DL have to be on BP day or PR day and what's the difference? It's just a question of how you want to stagger the programming. In the NLP, the "main" lifts are the SQ, PR, DL and so they tend to be done together and first, but you could certainly do the BP on DL days. As programming gets more advanced, there will often be a fixed 2x/wk DL day and then PR and BP will actually rotate in their association with the DL week to week. For some Masters athletes, the SQ isn't an option at all, and they only DL and they may actually do that first (before PR or BP) as it helps warm them up better.