Z-DOS was a version of Microsofts generic MS-DOS specifically written to run on the hardware of the Zenith Z-100 personal computer.[1]
The Z-100 used a 8086-family microprocessor, (the Intel 8088) but otherwise had a completely different internal architecture than the IBM PC.
At the time Microsofts MS-DOS wasn't specifically geared to any specific hardware platform, it could run on any system as long as it used a (16-bit) 8086 or similar microprocessor. A situation completly like the popular CP/M systems of the time, which used an (8-bit) 8080-family (8080, 8085 and Z80 among others) microprocessor. Just like CP/M this early form of MS-DOS relied on a, (CP/M like) platform specific BIOS, for hardware independency. There were dozens of such versions, each geared to a specific computer.
Only later the "MS-DOS" that we all know, became the rebranded version of the IBM specific version of MS-DOS, (known as PC-DOS) which (as "MS-DOS"), was sold to IBM PC (clone) makers.