• a semantic definition of each metadata element;
• whether or not a metadata element is mandatory, optional, or mandatory based on certain conditions;
• whether or not a metadata element may occur multiple times in the same record;
• constraints due to the organization of metadata elements relative to each other, e.g., hierarchical parent-child relationships;
• constraints imposed on the value of an element (e.g., free text, numeric range, date, or a controlled vocabulary); and
• optional support for locally defined metadata elements.