A file with the extension is essentially an installer package. In the official Sony ecosystem, when you bought a game from the PlayStation Network (PSN), you downloaded a PKG file. This file contained all the necessary data—executables, assets, and licenses—to install the game onto the console’s hard drive.
Searching for is a gateway to understanding how modern console preservation works. While the term “ROM” is technically inaccurate, the intent is clear: people want to play PS3 games on their own terms, whether via emulation or modified hardware.