here is a simple look at the most common leakage classes and how valves are tested to establish that leakage rate.

ANSI CLASS         Class III          
Maximum Seat Leakage     0.1% of rated valve capacity      Test Conditions
Air or water @ 50 - 125 F.               45 - 60 psig or Maximum operating differential pressure whichever is less
                     
                
ANSI                    Class IV              Maximum Seat Leakage    0.01% of rated valve capacity     Test Conditions    
Air or water @ 50 - 125 F.               45 - 60 psig or Maximum operating differential pressure whichever is less
                     
                
ANSI                     Class VI                  Maximum Seat Leakage     1" Port Diameter         
0.15 ml per minute or 1 bubble per minute     
Test Conditions    
Air or Nitrogen @ 50 - 125 F          50 psig or Maximum operating differential pressure whichever is less

Simplified Chart for most popular leakage classes. For complete specifications refer to ANSI/FCI 70-2

This should at least give you a start on leakage classifications.