You can define as many user counter variables as you like and you can define the starting number to be used for each variable, which makes it possible to use the variables across multiple projects. User-defined counter variables cannot be referenced. They simply provide consecutive automatic numbering, with no additional functionality.
If you use a counter variable inside topic or image captions you must only use it in topic or image captions. If you also use it in normal topic text the numbering will be wrong. (Variables in captions are evaluated in a separate pass when you publish.)