$Dollar Notation and [C]

I noticed while looking through some Out-Of-The-Box code that there is some dollar notation used that I couldn't find mentioned in the documentation. An example of this is $UID_Locality[C]:Bool$. My best guess is that it's a 'Change' comparison of $UID_Locality$ and $UID_Locality[o]$ and returns true if they are different. Is that right?