If you own a physical disc of a game, you can legally back it up in certain regions. For emulators like RPCS3, here’s the correct flow:
Note: On a modded PS3 (CFW/HEN), you can install unsigned/unencrypted PKGs. This allows you to install game backups, homebrew apps, and emulators directly from a USB drive.
In the world of emulation, there are two primary ways to play PS3 games:
In the strictest technical sense, a .pkg file is an archive format used by Sony to deliver content to the PlayStation 3. Think of it as a proprietary zip file or an installer executable (like an .exe on Windows). It was the standard vessel for distributing PlayStation Network games, DLC (Downloadable Content), patches, and system updates.
When you download a PSN game from the official store, you are downloading a .pkg. When the system installs it, it decrypts the archive and places the files into the appropriate directories on the console's hard drive.