metadata Container Specifications: Since XML is human as well as machine readable, it is the preferred method for specifying metadata containers; it is self-descriptive. The container specifications, however, don’t specify a single XML schema containing the complete set of metadata elements. Rather, they are frameworks of high-level elements that define extension points where specific descriptive, administrative, technical, and structural metadata can be embedded. This specific metadata is captured in extension schemas that define the specific metadata elements. It may be physically embedded or reference externally stored metadata.