Properly, Form Customization refers to the built-in feature of customizing the Resource editing forms. You can access this feature in the Manager Customization menu. It is somewhat limited in what you can do, for example you can hide and rename the default Resource fields, but you can't move them to other tabs or areas. And, of course, it only applies to the Resource forms.
Since the need occasionally arises to customize other forms, I've included tricks and code for plugins to similarly customize other forms, such as the User forms or the various element forms.