Originally, PhotoLab made no distinction between Master and Copy. Deleting the master would simply renumber the copies and no further harm came off it. But it was not easy to understand in the beginning, at least it wasn’t for me and I therefore posted a feature request to distinguish master and copy - and it got implemented a few releases afterwards.
If we don’t differentiate between master and copy, we’d need to understand that any image, be it master or copy, was just a meal cooked with the same ingredients (OOC image data) and different recipes. Once we understand this, we need not care about master and copy - because everything is just a copy and that the OOC image is never displayed anyways.
What name we see is just a label that could be changed.
Actual view would be copies derived from their respective recipes from the database/stored in the .dop sidecar file. All images only become real on export.
BTW: In DPL’s database, there are tables for the red (OOC) items and the blue (viewable) items. For DPL 7 on Mac, the tables are “ZDOPSOURCE” and “ZDOPINPUTITEM” and even though the same filename exists in both tables, they have different UUIDs.
Also see Make "master" deletable again / treat all copies the same