After you click the Generate BOA button in the Microsoft Word ribbon, a window will pop up with a list of all of the references used in your document. Beside each reference is a toggle to include or exclude the source. By default, all sources will be included.
Keeping sources on include will show the full case text of that reference in the Book of Authorities, or a placeholder for custom references. It will also generate a tab marker for any citation to that source in your document.
Any source that you deselect will be excluded, and therefore not appear in your Book of Authorities or have tab markers in your document. This can be useful for excluding legislation or any sources that are sufficiently hyperlinked elsewhere.