The CSO format is a compressed version of the standard ISO game file. It was originally popular because PSP memory cards were small and expensive.
Even today, on a Steam Deck or Retroid Pocket 4 Pro, the Tekken 6 CSO is the version you’ll find in “Top 10 PSP ROMs” lists — often mislabeled as a “special edition” or “Namco optimized release.” psp game tekken 6 cso file exclusive