An AS568 number is a three-digit suffix that identifies the o-ring's size. 

The SAE (Society of Automotive Engineers) published the original file that established these o-ring sizes. 

The AS in AS568 stands for Aerospace Standard. 

These o-ring sizes are standard throughout the o-ring industry and also in most military applications. 

AS568 is generally considered to be the U.S. size standard and there is no difference between the AS568A, AS568B or AS568C designations.