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- Attach an XML-attribute named “priority” with an integer value in the range 0 to 255 127 to each Attribute.
- Ensure that exactly one such Attribute has the priority value 0.
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Receiving User Agents who receive an entity with multiple attributes having the same name which do not have priority values defined MUST assign the priority value zero to the first such attribute (unless there is another such Attribute which has the priority value zero specified) and assign the priority value 255 127 to all other such attributes.
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Where multiple well known identifiers exist for a particular Entity type, the Entity format definition SHOULD permits permit the presence of multiple attributes with the same name to hold the potentially multiple identifiers for the same Entity.
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Where an Attribute has a priority, it MUST have an integer value in the range 0 to 255127. The priority describes the order of preference or reliability of the given value in the eyes of the message originator, with lower values indicating a preference over higher values, an identifier with a priority of 0 is the most preferred or reliable and one with a priority of 255 127 is the least preferred.
Within a single entity element where there are id elements present, there MUST be exactly one id element with a priority value of zero. This SHOULD be the identifier that the originator uses as the primary key for the entity being described.
Each id element SHOULD have a priority value which is unique among id elements in the same entity element. There is no intention to limit the total number of identifiers provided to 127 or any other number, this is why duplicate non-zero priorities are permitted.
Rendering Apps SHOULD use the identifier with the lowest priority which they are able to interpret. In the event that there are multiple identifiers with the same priority value which are the best available option for a rendering app it SHOULD use the identifier which appears first in the markup.
Composing Apps MUST NOT generate entities with multiple identifiers which refer to different entities.
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