Document Properties can be accessed not just through the limited number that are in QuickParts but also through Advanced Document Properties. I put a button for these on my QAT.
Modifying the Quick Access Toolbar (QAT) in Microsoft Word
You can use the properties built in or create Custom Properties. The ones not under QuickParts can be displayed in a document using the DocProperty field. Those under Quick Parts can also be displayed that way or inserted directly. The Document Properties under QuickParts are special. Those are Mapped Content Controls.
Repeating Data Using Document Properties Content Controls and Other Mapped Content Controls
If you type in one of those, it changes the property itself and updates throughout the document. You can make your own that will not appear on the menu. Typing in a DocProperty field will just delete the field replacing it with what you type. Typing in a Mapped Content Control changes the underlying property, and thus changes the content of any other document property content controls for that property in the document.
Word also has Document Variables which can be accessed only using vba. Like document properties they can be displayed in a document using a field but not changed using that field.