Custom XML Parts

This is a block of xml that can be saved in a document
This XML is saved in the documents data store
Every xml part should have an associated namespace

Binding to Content Controls

You can bind content controls to elements in a custom xml part
When a content control is bound to a custom xml part the contents of the custom xml part are displayed inside the content control in the document.

When the user edits the content control the corresponding XML element is automatically updated.

Updating with Word Open

This can be done from a VSTO document-level solution or an application level add-in

Updating with Word Closed

This can be done using the Open XML SDK
Open XML SDK can be downloaded from the Microsoft website

you could also use ServerDocument ?

Adding

Binding

Binding to content controls
The XMLMapping property can link the content to a content controld