Anatomy of ``Composite'' enclosure

Composite enclosures use the ``Enclosures'' (plural) element name and have the following attributes:

Attribute	Mandatory	Description
kind	optional	kind of enclosure - specifies the relationship between sub-elements (see below)
rel	optional	This attribute describes the relationship of the enclosing element (``item'', ``atom:entry'' or another ``Enclosures'') to this enclosure. The value of this attribute is a space-separated list of link types.
Content-ID	optional	Unique identifier of this enclosure (must be unique within ``item'' or ``atom:entry''). (``cid'' URL schema[11] could be used).
description	optional	Human-readable brief description of the enclosure contents

Composite enclosures should have one or more nested ``Enclosure'' or ``Enclosures'' elements.

The ``kind'' attribute specifies the relationship between sub-elements of this composite enclosure. Defined values are:

parallel:

This value recommends the display of all sub-elements simultaneously on both hardware and software that are capable of doing so. However, composing agents should be aware that some readers may lack this capability and will therefor show the sub-elements serially instead.

mixed:

This value specifies that sub-elements are independent. The order is important and, if possible, they should be shown in the order in which they appear.

alternative:

This value specifies that all sub-elements represent alternative versions of the same information. Systems should recognize that the content of the various sub-elements are interchangeable. Systems should choose the "best" type based on local environment and references, in some cases even through user interaction. The first element is considered to be default or preferred, and the order of remaining elements is not significant. All elements including the first could have a ``quality'' attribute which could be used to decide which element to show (see section 3.4). In general, it is recommended that preferred element be used (if it is in format client software understands) even if there are elements with a higher ``quality'' factor. It is assumed that the feed author has choosen it as as reasonable compromise between quality and required bandwidth.

Additional user-defined values of the ``kind'' attribute are allowed, but their name should start with an ``x-'' prefix. For example, ``x-random''. When an RSS reader encounters a user-defined enclosure ``kind'' which it does not know how to handle, it should treat the enclosure as ``mixed''.

If ``kind'' was not specified, ``mixed'' is implied.