The marks on the thread dial (there are usually 8, 4 larger and numbered, and 4 smaller unlabeled in between the number marks) are starts for various thread pitches (tpi). If your chasing (cutting) a thread that is divisable by 8 (8,16,24 etc) you can start (close half nuts, pull the lever) on any mark. If it's a odd (5,7,9 etc) or even (2,4,6 etc) then start on a numbered line. If it's a half thread (4 1/2, 3 1/2 etc) then start on either 1 or 3, or 2 or 4. Finally (it's already more info than wanted) if it's a quarter thead (2 1/4, 1 1/4 etc) start on the same number every time. If it's to much to bother with, then just always start on the same number. Some lathes don't have the dial and so the lever can only be engaged in one spot per revolution of the lead screw. This makes for slower, but safer threading. Hope that answers the question.
Pat