XMP Specification September 2005 17 XMP Data Model Property Values 2 XMP supports three types of arrays: unordered, ordered, and alternative, described in the following sections. Unordered Arrays An unordered array is a list of values whose order does not have significance. For example, the order of keywords associated with a document does not generally matter, so the dc:subject property is defined as an unordered array. In the schema definitions, an unordered array is referred to as a bag. For example, dc:subject is defined as “bag Text”, meaning that there may be multiple text-valued subjects whose order does not matter. Ordered Arrays An ordered array is a list whose order is significant. For example, the order of authors of a document might matter (such as in academic journals), so the dc:creator property is defined as an ordered array. In the schema definitions, an ordered array is referred to as a seq. For example, dc:creator is defined as “seq ProperName”, meaning the order of the creators matters and each creator value is a proper name (defined elsewhere). Alternative Arrays An alternative array is a set of one or more values, one of which should be chosen. In the schema definitions, an alternative array is referred to as an alt. For example, xmp:Thumbnails is defined as “alt Thumbnail”. There are no specific rules for selection of alternatives: in some situations, an application may make a choice in others, a user may make a choice. The first item in the array is considered by RDF to be the default value. A common example is an array that contains the same logical text (such as a title or copyright) in multiple languages. This is known as a language alternative it is described further in “Language Alternatives” on page 18. Property Qualifiers Any individual property value may have other properties attached to it these attached properties are called property qualifiers. They are in effect “properties of properties” they can provide additional information about the property value. For example, a digital resource representing a musical production might have one or more authors, specified using the dc:creator property, which is an array (see the figure below). Each array value might have a property qualifier called ns:role, which could take a value of "composer" or "lyricist" or possibly other values. NOTE: At this time, only simple properties may have qualifiers, and the qualifiers themselves must be simple values (not structures or arrays). This is because of limitations in early versions of the Adobe XMP Toolkit.