A family of Microsoft word processing software products for creating web, email, and print documents.
See the following video
Warning! That video involves the use of the Master Documents "Feature" .
You never want to do any editing in the Master Document that is created.
The sub-documents are fine as actual documents.
What you are discussing is the automatic naming that takes place when you create the sub-documents.
When a new Word document is saved, the default name is the first line of text up until punctuation unless there is a title property in which case it is the title property up to punctuation. I do not have a clue what would happen with a Master Document that had a title property but suspect it would be disregarded. (Confirmed, a title property in the master document is disregarded in naming the subs.)