A version is an ISO file that has been ripped and compressed to reduce its size significantly. Typically, these versions are reduced to roughly 200MB to 500MB .

The PSP used UMD discs with a lot of "dummy data" (empty padding) to speed up laser reading. Compression tools strip this padding away and condense video/audio files. When you run a compressed CSO file in PPSSPP, the emulator decompresses it on-the-fly.

🔍 "Midnight Club 3 DUB Edition CSO 200MB" "MC3 Highly Compressed PPSSPP Android"