Marking Vocabulary
Vocabulary words may be marked as "Known" or "Tracking".
Known words are words that you are already familiar with. Words marked as known are excluded from frequency lists. Marking a word as known increases the progress bars for all series and volumes containing that word.
Tracked words are words you do not know yet, but you want to track. One use for this is to track words you have added to Anki or another SRS application, so you know at a glance whether you already have a flash card for a word. Words marked as tracking factor into the "tracked" portion of the known words progress bar.