Upvote:6
A permit allows you to enter Thailand, and is valid until a certain date. If you are staying for longer than 90 days, you must notify the immigration bureau every 90 days. This notification is not an extension of your permit's expiry date.
The difference between overstaying and overdue is that overstaying relates to you and your presence in Thailand, while overdue relates to the notification:
If you are late submitting the notification (this must be done no later than 7 days before the end of the 90 day period), the notification is overdue.
If you stay beyond your permit's expiry date, you are overstaying.