These standards take a VERY conservative approach towards reserving variables for functional use. In most any programming environment it's up to the developer to declare and use variables for the implementation of their solution.
There are, however, common enough cases where it just makes sense to reserve a word for variable use. Within SQL and various other languages, there are reserved words which cannot be used because of their special importance. This section provides best practices with regards to reserved $localVariables and $$GLOBAL.VARIABLES.
There is currently only one suggested critical local variable which is reserved. That variable is the $error variable. As error handling and error logging is such a common practice in most software, the Error Handling best practice reserves this variable. It extends this variable to offer a few other variables should you choose to extend it. These include $errorLevel and possibly $result, dependent upon your election to use the full suite of functions.
None currently