The Webel '$opt$' option "help holder" convention

For reference, the related Webel convention policy note (explored through examples in later slides, suggest don't visit this link yet): Dr Darren says:

This is an absolutely super tried and tested convention, and you don't need the Webel libraries to benefit from it. I even use it (and without depending on the Webel libraries) in standalone Mathematica Notebook mini projects all the time. Even just using '$opt$' for prompting and 'def$opt$' for keeping defaults nice and DRY within a Notebook are handy.

This '$opt$' convention is used heavily throughout all of the project application examples in this trail and throughout nearly all Webel examples of Mathematica projects, some of which have hundreds of options.