The Free On-line Dictionary of Computing (30 December 2018):
Plug Compatible Manufacturer
(PCM) A manufacturer of equipment that some other
manufacturer's system can identify and work with. The PCM's
device replaces the original manufacturer's.
Most PCMs competed with IBM. PCM devices normally offer a
cost-performance benefit over the original device. For
example, several PCM versions of the Direct-Access Storage
DeviceIBM 3350 offered twice the storage and improved data
access (dual port).
Plug compatible devices include replacement CPUs, such as
the Hitachi 7/90 series (which could be substituted for IBM
3090 series processors), I/O subsystems, and dumb
terminals like the IBM 3270.
[Used outside mainframe market?]