neowin[.]net/news/microsoft-quietly-updated-its-official-lightweight-windows-11-validation-os-with-new-iso/
WinPE, short for Windows Pre-installation Environment, is what is used by ~99% of the bootable USB sticks you create in backup & partitioning apps. You can only legally get it from Microsoft -- you have to 1st download & install the ADK, then create a working folder with the WinPE files, then use further commands to add it to something like a USB stick. WinPE just gives you a command prompt, but it will run some [usually simpler] apps in their typical Windows GUI. And for some reason WinPE will restart after 72 hours [tradition? Because that's the way it's always been?]. You can run a full copy of Win11 without activation, & it will have just a few minor restrictions, e.g., you have to use a password to log in rather than a PIN, so to me the 72 hour limit is really a bit silly.
And that 72 hour limit is really the only reason I can think of for Microsoft to release their Validation OS. Like WinPE the OS itself is packaged in a .wim file, a type of image file developed by Microsoft that's something like a specialized Zip file. You mount that .wim file to add stuff like drivers & apps, then save it & unmount the file. I *think* WinPE comes with more included in its base image -- you have to add quite a lot to the Validation OS if you want something more or less universal, and that step is a bit of a pain. The WinPE & Validation OS .wim files are close to the same size -- the Validation OS might be a little bigger, depending on what you add to it. To make a bootable USB stick using the Validation OS you 1st create a WinPE bootable USB stick, then replace the boot.wim file with the Validation OS .wim.
WinPE is more capable -- it's quicker to start & will run some apps [e.g. Notepad] in their normal window, which Validation OS will not. But if I'm correct and the Validation OS does not have a time limit, it would be the better choice for running programs beyond that 72 hour limit, e.g., stress testing.