Metadata model instance. The metadata JSON schema consists of a header and a body. A metadata body is an array of metadata groups; each group contains attributes (non recursive fields) and/or loops (recursive fields constituted by one or more attributes). Each attribute is a JSON object described by a set of properties to define its type ('STRING', 'INTEGER' or 'FLOAT'), name, if a value and a unit value is required, an optional custom value. A list of possible values and possible units can be stored as lists. If the attribute type is numeric ('INTEGER' or 'FLOAT') users can specify minimum and maximum allowed values using the lowerBound and upperBound properties. It is possible to name attributes using ontology terms and storing the corresponding URI in the property ontologyURI. If the Boolean property hasTableConnection is set to true and a tableConnection term is defined, the set of possible values is recovered from the XTENS repository internal database. The instances property contains all the attribute values and units assigned by the user.