CP/M is an operating system for microcomputers with Z80 and 8080 CPU, which was very popular in the 80s. Thanks to modularity, it is easily adaptable for different hardware platforms. CP/M was available for computers from many manufactures. Programmers could write universal software and users could execute it on every computer with CP/M. A lot of programs appeared, many of which were considered professional at that time. Some of them (like Turbo Pascal, Wordstar, dBase) became real hits.
Computers with processors other than Z80/8080 didn't have the ability to run CP/M. Their users could only dream of the popular CP/M programs. So, add-ons containing Z80 processor were manufactured, allowing tu run CP/M on such machines. For 8-bit Ataris there were ATR8000 or prototypes like the 1060 or CP/M card for the 1090 module. The Atari computer acts as a terminal in such configuration.
This page applies to the Indus CP/M system which runs on disk drives: Indus GT, LDW Super 2000 and CA-2001. Until now, such feature of the two latter ones has been unknown (even by the manufacturer ;)). In case of Indus GT, the RamCharger card, a real vaporware has been required.