Setting Parameters for all email templates?

Brand new Acronis Access user here, and I've got what I hope is a very simple question:

When customizing email templates, I'm shown a list of available parameters I can modify, but the documentation doesn't say where I actually declare these parameters...

Is there some central file that's referenced?  Or do I need to declare them somehow in the body of the template?  If there's a central way to declare them (setting product_name to be WhateverName, for example), I'm hoping that will apply to all templates?

Any info would be appreciated, thanks!