I downloaded \"Office 2010 Add-In: Icons Gallery\" that is docx file with two backstage tabs that hold icons.
How can I extract the icons or use them in my application?<
I suppose the first answer meant to say, that the icons are only meant to be used within Office; that is with Office add-ins and such.
They can be referred with Office Fluent UI definition XML for example. So the distribution form serves as visible "Ahh I want that icon in my add-in" reaction and then choosing the specified ID to use in "imageMso" attribute.
Like this (see proper full XML in Office Fluent UI customization)