Does a XSD schema define the minimum or maximum structure of an XML doc?

Assume I have an XML doc and the referring XSD schema.

Can I add now additional elements and additional attributes (for existing 
elements) in the XML doc without getting the XML doc invalid?

That would mean the XSD represents the minimum required content of an XML doc
which can be extended by multiple own parts.
I am not talking about minoccur or maxoccur but the pure declaration of elements
and attributes.

Ben