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?
Yes, $ColumnName[C]:Bool$ returns if a column has changed. It uses the object metadata to return the boolean and is therefore faster than writing something like $ColumnName[o]:String$ <> $ColumnName:String$
This is documented here