The original 1981 arrangement between IBM and Microsoft was that Microsoft would provide the base product and that both firms would work on developing different parts of it into a more powerful and robust system, and then share the resultant code. PC DOS and MS-DOS were to be marketed separately: IBM selling to itself for the IBM PC, and Microsoft selling to the open market. However, at no time did IBM acquire the ownership of the source code of the operating system for its own PCs.
PC DOS remained a rebranded version of MS-DOS until 1993. IBM and Microsoft parted ways. MS-DOS 6 was released in March, and PC DOS 6.1 (separately developed) followed in June. QBasic was dropped and the MS-DOS Editor was replaced with E. PC DOS 6.3 followed in December.
PC DOS 7 was released in November 1994. The REXX programming language was added, as well as support for a new floppy disk format, XDF, which extended a standard 1.44 MB floppy disk to 1.86 MB.
The most recent release was PC DOS 2000, which found its niche in the embedded software market and elsewhere. It was based on PC DOS 7, and corrected issues with the Year 2000 problem. Branding of PC DOS 2000 included the phrase "includes PC DOS 7".