Structural metadata describes the physical and/or logical structure of digital resources; it expresses the intellectual boundaries of complex objects and can be used to describe relationships between an object's component parts. Structural metadata is commonly used to facilitate navigation and presentation of complex items by defining structural characteristics such as pagination and sequence, and, like METS, can be used to aggregate related metadata.

Structural metadata should be used when a digital resource is divisible into component parts and the components are useful in-and-of themselves. For example, a digital book may be logically structured into chapters but "physically" exist as images for each page.  A chapter, which is an aggregation of page images, is considered an intellectual entity (as are the individual page images). The structural metadata should be used to reflect how those component files are logically grouped and, ultimately, how they should be perceived.

Another example where structural metadata is appropriate is a slide presentation containing a visual stream and an audio stream. Both streams are intended to run concurrently in the presentation but both may be useful on their own. The structural metadata would serve to 1) identify and separate the streams and 2) ensure proper cueing and synchronization between the streams.   

Common structural metadata schemes are Metadata Encoding and Transmission Standard (METS), MPEG-21: Digital Item Declaration Language (DIDL), and Synchronized Multimedia Integration Language (SMIL).

Choose well-known and well-documented Structural Metadata scheme.

When selecting a structural metadata scheme it is best to choose one which has an open and accessible specification. It is best to avoid closed or proprietary schemes which limit sharability and usability of the items.  It is best to choose a scheme that is commonly used for the type of digital objects being modeled and which has been proven in practice. Unless there is no existing standard which fulfills the modeling requirements of the object type it is not suggested that a custom structural scheme be developed. Utilizing pre-existing standards ensures that items are more sharable between institutions and rendering/display systems. You can also leverage documentation, utilities, and best practices from the chosen standard's community of practice. It is advisable to use a structural metadata scheme with a public serialization definition such as an XML Document Type Definition or Schema Definition whenever possible.

Choose a Structural Metadata scheme (or profile) appropriate to the class of items being modeled and their intended use.

When selecting a structural metadata scheme for a particular project it is best to choose a scheme appropriate to the type of digital items being modeled and the proposed usage of those items. For example, file-sequential or paged text objects may be best marked up in METS due to the availability of page-turning applications that understand METS.  While a multi component audio-video presentation with synchronized components may be best encoded in Synchronized Multimedia Integration Language (SMIL) which is renderable by media players.

Clearly define a usage profile of the chosen structural metadata scheme outlining local conventions and practices.

It is best to create a usage profile defining local practices. This profile can include instructions concerning which elements are to be used and how they should be used, any value enumerations required in specific elements, naming conventions, controlled vocabulary usage, or conventions related to identifier generation and usage. If following a pre-existing profile, this should be indicated.

Develop written guidelines / procedures outlining the creation or modification of SM according to the item's work flow.

If a project has procedures for creation of structural metadata, these procedures should be documented.  For example, if a project uses a particular XML editor with defined templates for data entry then those templates should be well documented and made available to all metadata creators. If structural metadata is created by an automated process such as a script then usage information about that script should be available to all who will use it.

Validate all created metadata against its publically posted schema definition or a locally held schema definition.

When creating structural metadata it is best to validate against a serialization definition (XML Schema). Whenever possible the "officially" published definition should be used. For example, if METS is used, validation should be done against their published XML Schema for METS. If using a locally developed schema definition, ensure that all that create structural metadata have access to and validate against this local schema.