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Qualcuno di voi può aiutarmi nella spiegazione degli xml schema

Non riesco a capire l'utilizzo di questo concetto..

These datatypes are lists of whitespace-separated items. The type of these items (called
the item type) is defined during the derivation process (which we will see in the next
chapter) and list datatypes can be derived from any simple type. Three predefined
datatypes are lists (xs:NMTOKENS, xs:IDREFS, and xs:ENTITIES). For all the list
datatypes, the items must be separated by one or more whitespaces.
xs:NMTOKENS
This is a whitespace-separated list of xs:NMTOKEN. Each item of the list must be in
the lexical space of xs:NMTOKEN.
xs:IDREFS
This is a whitespace-separated list of xs:IDREF. Each item of the list must be in
the lexical space of xs:IDREF and must reference an existing xs:ID in the same
document.
xs:ENTITIES
This is a whitespace-separated list of xs:ENTITY. Each item of the list must be in
the lexical space of xs:ENTITY and must match an unparsed entity defined in a
DTD.