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To understand content aggregation in Fedora, you need to be comfortable with two terms:

1. Digital Object — – This is the basic unit for information aggregation in Fedora. At a minimum a digital object has:
   1. An identifier or PID (Persistent Identifier). The PID provides the key by which the digital object is accessed from the repository.
   2. Dublin Core metadata that provides a basic description of the digital object.
2. Datastream — – A component of a digital object that represents a data source. A digital object may have just the basic Dublin Core Datastream, or any number of additional Datastreams. Each Datastream can be any MIME-typed data or metadata, and can either be content managed locally in the Fedora repository or by some external data source (and referenced by a URL). When you create a new Datastream in a digital object, you assign it to one of four types, or control groups, depending on the nature of the data that it represents.
   1. Managed Content (M): Datastream content is stored and managed within the Fedora repository's persistent storage. The content can be any MIME type including XML.
   2. Inline XML (X): A special case of M, restricted to well-formed XML. In this case, the Datastream content is stored as part of the XML structure of the digital object itself and is thus included when the digital object is exported (e.g., for archival purposes).
   3. Externally Referenced (E): Datastream content is external to the Fedora repository and is referenced by a URL that is recorded within the digital object. The content can be any MIME type including XML.
   4. Redirected Content (R): Like E, but Datastream content is delivered to the client without any mediation by Fedora; i.e., via an HTTP redirect. You should use this Datastream type when the external content is a web page with relative links or it is streaming audio or video. The content can be any MIME type including XML.

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