The most recent version of this documentation can be found at https://docs.dyalog.com/20.0/.
CMD_PREFIX and CMD_POSTFIX
These parameters defines strings within which operating system commands specified as the arguments to ⎕CMD and ⎕SH , and )CMD and )SH , are wrapped. Its purpose is to run the command arguments under a non-standard command shell. This applies to Windows only.
See Windows Command for implementation details.