Optional [element], Single Occurrence, with Simple Type [inline-Item-Key] localName is [Key] without namespace A list of uniquely keyed items and a list of references to the keys - the [refer] attribute is used to specify the [key] constraint to which refers the [keyref] - [keyref] Instance constraint example from Eleusoft Application Documentation Required [element], Multiple Occurrence, with Complex Type [Item] localName is [Item] without namespace Required [element], Multiple Occurrence, with Simple Type [inline-Form-Reference] localName is [Reference] without namespace A list of uniquely keyed items and a list of references to the keys - the [refer] attribute is used to specify the [key] constraint to which refers the [keyref] - [keyref] Instance constraint example from Eleusoft Application Documentation