Commands that return a value into the command line were called “evaluated commands” in the original Multics shell, which used square brackets where Unix uses backticks. (source) In summary, rc. d stands for “run commands” at runlevel which is their actual usage.

What is RC Linux?

In the context of Unix-like systems, the term rc stands for the phrase “run commands”. It is used for any file that contains startup information for a command. It is believed to have originated sometime in 1965 at a runcom facility from the MIT Compatible Time-Sharing System (CTSS).

What is an rc script?

The rc script

When init enters a runlevel, it calls the rc script with a numeric argument specifying the runlevel to go to. rc then starts and stops services on the system as necessary to bring the system to that runlevel. Though typically called at boot, the rc script can be called by init to change runlevels.