While the CSO format is the most widely compatible, the PSP homebrew community has developed several "top" alternatives over the years to balance storage space and loading speed:
You're looking for information on the "CSO PSP Archive Top". Here's what I found: cso psp archive top
format, meaning a CSO can be decompressed back into its original ISO bit-for-bit. Compression Levels: While the CSO format is the most widely