Prefixes as placeholders
Namespaces in XML treats namespace prefixes purely as stand-ins for URI values, with no meaning or weight of their own. Early XML tools often discarded prefixes completely. More recently, prefixes have received more respect (and preservation) as tools for making XML more human-readable, but they remain locally-declared pointers to URIs.
Prefix-URI mapping
Because the namespace was defined in an xmlns:prefix attribute, the prefix portion of the attribute name is associated with the URI in the attribute value.