OVA8 (about credit control) - "Highest dunning level"

At OVA8 (about credit control) we have "highest dunning level". I can read the explanation below but I couldn't understand why "highest dunning level" stands for. Can anyone explain me?

Credit check against highest dunning level allowed

Indicates whether the system carries out a credit check based on the highest dunning level.

Use

With this type of credit check, you specify the highest dunning level you want to allow in the adjacent field. The dunning level is tracked and stored in the credit data in the customer master record. If this level is exceeded during order or delivery processing, the system carries out a credit check.