CSO is used to compress dumped PlayStation Portable UMD games, and is an alternative to the DAX compression method. It is also sometimes called CISO.
CSO files are compressed ISO image files. They are often used as a medium to play psp games. CSO files may include up to nine levels of compression. While the highest levels of compression can introduce slowdown and lengthy load-time in software which relies heavily on disc streaming, even the lower levels are capable of substantial compression.
In addition to being used for compressing PSP UMD games, the PSP port of PicoDrive supports compressed Sega Mega-CD disc images in the CSO format.