I didn't find a clear explanation anywhere but it was written here at the Thailand immigration bureau website:

• Overdue notification is not accepted via internet. Applicant must contact the nearest Immigration Office in person.

It was further written in the same webpage:

The applicant (or authorize someone to file for you,only in case of not overdue), must come to the nearest Immigration Office or Branch Office in your residence area. The applicant will need to bring the following required documents:

• Your Original passport
• Your departure card (TM.6)
• Previous receipt of notifications (if any)
• Completely filled in and signed notification form TM.47

My problem

It is not clear to me how an overstay and overdue(staying) in Thailand differ one from the other.

It is especially unclear to me if "overdue" is a code name for what I can define:

A special extension of generally any visa, due to some special unexpected event, such as a legal issue or acute, urgent medical condition (such as a car crash) or in extremely rare cases some other reason for the consideration of an immigration official or the king (whatever that might be).

My question

What is the difference between overstay and overdue in Thailand?