Doubtful
Users who have a balance below zero for more than 30 days but less than 90 days are considered "Doubtful" and are placed on a separate list.
It is recommended to work on them in order to find out the reasons for arrears and their plans for using the services.
All such users will have a special mark in the user lists.
The limit of days (30-90) is a customisable.
