It refers to the template that is both open for editing and the active document. I.e., if it is open but you are in a different document, it does not count.
What are you trying to add? QAT, Building Blocks/AutoText, Macros, Building Blocks? When you are trying to save these you are in a dialog box that generally has a dropdown.
Microsoft uses different terms in the dialogs for the storage location but it is not that tough.
You are welcome to share a screenshot showing what you are (not) seeing when you are trying to save if you want.
How to attach a screenshot in this forum.
Here is a link to a handout I prepared for a talk I'm giving next week.
My list of links on QAT and Ribbon Modification