This field is located on the Main tab of Customer Maintenance when you click the POS hyperlink in the far right column. It allows the you to govern whether or not a given customer's transactions receive the past due warning based upon how past due they are.
Blank = Past Due Warning based on Option ID 420 (default)
1 = Warn when 1 or more days past due
2 = Warn when 31 or more days past due
3 = Warn when 61 or more days past due
4 = Warn when over 90 days past due
If the customer’s Charge Allowed field is not P AND they are past due, and if this field is not blank, the system determines whether or not to give the past due warning based upon this field. I this field is blank, the warning is based upon how Option 420 is set for the store.