More about AutoText (ignore info about Building Blocks, Content Controls, and Quick Parts when using menu versions of Word):

• AutoText
• AutoText Fields - Incorporating AutoText or other Building Blocks in Templates
• AutoTextList in the Modern Age
• AutoTextList field - How to add pop-up lists to any Word document, so you can click your way through changes in seconds
• Where can Building Blocks (including AutoText) be stored? (AutoText in menu versions is only in templates, not in a "Building Blocks folder."

If you use the AutoTextList field, what determines what goes into the menu is the Style used for the text when it is saved. This field can be restricted to a particular style and you can make up custom styles just for that purpose.