Configuration file operators

I'm working on modifying some commands in our configuration, and had a couple questions.

I've looked online to better understand what the "$" does within these files. For example:

     MS_CONFIG_DIR: $(MS_CONFIG_DRIVE)Microstation/Config/

I understand that the command line is referencing a previously established variable (MS_CONFIG_DRIVE) but I don't understand what the $ is doing. Is it telling microstation to look for a previously established variable, named after the $ and between the two parenthesis?

I'm also looking to understand the difference between the ":" and "=" within the config documents.

I've seen:

     MS_CONFIG_DRIVE: Y:/

as well as:

MS_DESIGNMODELSEEDNAME = 2D Design

On the surface, it seems like both operators work in a simlar fashion.

More specifically, I was attempting to set up an if / then statement to reference local files if the network drive was unavailable:

     MS_CONFIG_DRIVE = C:/Program Files (x86)/Bentley/MicroStation V8i (SELECTseries)/

     %if exists (Y:/)

     MS_CONFIG_DRIVE = Y:/

     %endif

But first I had tried:

     MS_CONFIG_DRIVE: C:/Program Files (x86)/Bentley/MicroStation V8i (SELECTseries)/

     %if exists (Y:/)

     MS_CONFIG_DRIVE: Y:/

     %endif

Then first example worked correctly, but the second one did not. I would appreciate any advice. Our company no longer employees the CAD guru who set up our files, and I am working to better understand how this all fits together.

Thanks.