Back in Jimmy Carter's days in The White House, the USDA relaxed beef grading standards. USDA Choice was widened considerably to include what were before,the upper echelons of USDA Select beef.
Enter the 1980's...some brilliant cattleman came up with the idea of branding Angus beef under the name of Certified Angus Beef or CAB. The idea was to designate the upper 30% of Choice (what WAS Choice before it was diluted) Angus for this label.
Genuine USDA Choice CAB is still culled from the top end of the grade. The conformation, quality and marbling is a cut above average. CAB does market USDA Prime but not USDA Select. CAB is usually more expensive (25% +/-) and in my opinion, generally worth the difference.
Be aware, a lot of branded beef now contains the word "Angus". Only CAB can display the CAB logo.