Turnup is right the scoring systems are Inward Gauging (the highest score taken for the ring which is cut) used on 50 and 100 yard Prone target shooting, and Outward Gauging as used on 15,20 and 25 yard prone shooting for NSRA competitions. I think the other method is called biased and as stated uses the centre of the hole not the edge of it thus making it fairer in competitons such as black powder where a number of varying calibres may be acceptable for the competition rules.
ATB Martini