What does "Class 10" and "Class 20" mean when selecting overload relays?

What does "Class 10" and "Class 20" mean when selecting overload relays?

"Class" is an important rating in regards of overload relays(OLRs). The "Class" specifies the maximum amount of time the overload will allow 6x (600%) of the full load rating of the OLR before tripping.

For example, a Class 20 OLR will allow 600% of its rated full load current for up to 20 seconds before tripping.