These variables are used to late bind values to fields in unattend.xml or kickstart file where the value is not known until execution time (e.g. subnet mask since it is calculated based on value for subnet mask which can be a property ) or for some reason the values should not be shown ( e.g. Admin password). These variables are evaluated only in unattend or kickstart files at execution time. It wouldnt make sense to use them in post-install script or any other place. Also, you cannot explicitly specify values for these variables.
What is your use case, I think you can use properties, in most cases it will serve your purpose.
The use case for this question was to see if there is an equivalent to the LINUX_USERS_FILE_ENTRY for Windows as I wanted to get and store the name of the person performing the build in the registry of the server being built.
I know that I could write a script to extract this from the users file, if I get to it before the ACL's are pushed, but it would be much easier to use a built-in variable or the same approach that BMI uses.
Some detailed (or any) documentation would be useful on these variables