Nextor is a disk operating system for MSX computers. It is actually an enhanced version of MSX-DOS, with which it is 100% compatible, and it identifies itself as MSX-DOS 2.31 to MSX-DOS aware applications.
This version is the first one incorporating a much requested feature: the disk image emulation mode, that allows to boot the computer using disk image files as if they were real floppy disks, allowing to play many games that do not work in the regular Nextor/MSX-DOS 2 environment.
Link: Nextor Website
Source: MSX Resource Centre