You may have sent overload relays with different Class ratings.  Class 10 and Class 20 tend to be the most common there is also a Class 5 rating and a Class 30.  So what does the difference mean? Simply put the Class rating specifies in what amount of time the overload will trip after it is in an over amperage situation.  For example, a Class 10 overload will trip in 10 seconds or less (depending on the percentage variation of AMP pull) and a Class 20 will trip in 20 seconds or less.

So why is this important and how do you know which to choose?  Class 10 is the most common as it provides a high level of protection for the motor, which is the main function of a contactor with overload relay protection.  Class 20 is used in situations where you want to avoid nuisance tripping.  Let's say you have a motor with a high inrush that maintains a AMP pull that can last 10-15 seconds.  If you use a Class 10 overload relay, it would trip even though there is not potential damage to the motor.  A Class 20 overload would allow for a little extra time for that motor to level it's AMP draw.

So to sum up, Class 10 overloads are the most common as they provide a high level of protection for a motor while Class 20 overloads are used in situations where nuisance tripping is an issue.