Have you ever been frustrated trying to find *all* the possible variables used in the config files and init scripts in a RHEL or derivative distribution?
I was working on a RHEL setup to configure a "dummy0" interface with very specific requirement: this interface shall *not* be managed by the NetworkManager.
The RHEL Networking PDF (official doc) does not mention NM_CONTROLLED! After much searching, I found the answer in this Ars Technica discussion forum https://arstechnica.com/civis/viewtopic.php?t=767128
They are all defined in a file that goes by the following name. /usr/share/doc/initscripts-xxx/sysconfig.txt (where xxx is the initscripts version installed by the distro)
I hope you find it useful in your journey as a Sys/Net Admin working with RHEL and/or derivative.
Cheers, -- akk