"It's the best mechanisme for saving data as Metadata in the document "

Is it not really the ONLY mechanism for storing persistent data in a Word doc? That is the purpose of the Variables collection. From Help:

"Document variables are used to preserve macro settings in between macro sessions."

Although, really, "settings" is too narrow a word. And not specific enough. Document Variables store strings.

Is there another way to hold persistent data? I mean, actually in a document.