Select text, press a named category, and it is highlighted in that category's colour — ordinary Google Docs highlighting, with a legend. The panel counts the passages in each category, and Extract passages writes them, grouped, at the end of your document. Pro marks every match of a word or phrase at once, saves your own palette across documents, and shows how much of the document each category covers.
One permission, nothing else. It only ever touches the document you have open, runs entirely inside Google Apps Script, and sends nothing anywhere — no server of ours, no database, no analytics. Your highlights are part of your document and survive without the add-on.