The 15 ball has no special significance except that it's worth more points than anything else. So 61 points (or three-fouling your opponent) wins the rack, and you play to a certain number of racks. Spots are given out as points, not balls, so instead of playing 61-61 (even), you might play 75-46 or 68-53. oKok.

billiard rules 15 balls