The concept of reserved words has existed in most all programming languages. Within Claris/FileMaker itself there is a small collection of reserved words (see the TextStyleAdd () function and its styles parameter). The SQL query language has a long list of reserved words.
Following this convention these standards promote the following.
$$GLOBALS can't truely be private right?
Correct. However, the use of the tilde to indicate the "private" nature of code or values is not based on a language construct. Claris/FileMaker does not have the concept of private vs. public scope when it comes to code. Hence a $$~DOMAIN.IDENTIFIED.GLOBAL should only be accessed by the DOMAIN which manages it. It's a respect issue, not a technical one. Define your DOMAIN within the Reserved words script and don't mess with the variable except through the script/code which controls it. Aside from that, the use of these types of variables is quite rare and should be used sparingly.
It is highly suggested you create a script named "Reserved words" and maintain that script within the top level and the first ten scripts. It should be unchecked for inclusion in the Scripts menu and will primarily include comment steps used to document any "reserved words" defined by your solution.
# The following is a list of the reserved words within this solution.
# $$CLIPBOARD is a reserved temporary global location. NEVER expect to persist any data within it!
# CLIPBOARD is a reserved global field possible within any table.
# FOUNDCOUNT a reserved unstored calculated field set to Get ( FoundCount ) for any given table.
Every FileMaker solution ends up using a collection of both local $variables and global $$VARIABLES. While local variables are commonly used within the context of scripts only. They can be used in other areas of FileMaker development. When it comes to global variables there are certainly best practices for managing Reserved Variables.