What is the difference between the two types of 1951 USAF Glass Resolution Targets that you offer?

We offer 2” x 2” and 3” x 3” 1951 USAF Glass Resolution Targets in both standard and high resolution versions. Standard resolution targets range from Group -2 to Group 7 and Element 1 to Element 6; they can resolve up to 228 lp/mm. High resolution targets range from Group -2 to Group 9 and Element 1 to Element 6; they can resolve up to 645 lp/mm. High resolution targets can resolve much smaller features than their standard resolution counterparts.