Example of how the Office Add-in uses Word and Excel custom property fields

After you save an Office document with the Add-in, go look at the File/Properties menu item and then look at the Custom tab. That's where the Add-in stored all the "metadata" about the document. The document number is here, the categories, the division, the document type, etc.

The bonus to this is that you can display these pieces of information in the document, for example in the footer of the document to reference the project and the document number. Very handy to have this at hand.