# NMOS Source Attributes
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This Source Attributes parameter register contains extensible attributes and their permitted values which may be used in Source resources within the [AMWA IS-04 NMOS Discovery and Registration Specification](https://specs.amwa.tv/is-04).

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## Criteria

- Each entry MUST be associated with an existing defined [NMOS Format](../formats).
- Each entry MUST define or refer to an existing unique attribute name within the scope of a Source.
- Each entry MUST have a short description.
- Each entry MUST identify any AMWA Specifications and versions from which it is valid.
- Each entry MUST define all permitted values for the attribute, either via an explicit list, well-defined references or both.
- Entries MAY be used to expand the set of permitted values associated with an existing extensible attribute.
- Each entry MUST link to a schema definition held within this repository (unless covered by a schema within the original specification).
- Additions and updates to this parameter register are to be submitted via a Pull Request (PR) according to the [General Procedures and Criteria](../common/).

As noted in [IS-04 v1.3](https://specs.amwa.tv/is-04/v1.3.2/docs/Behaviour_-_Nodes.html#all-resources), new Source attributes may be defined here as opposed to requiring a new version of the specification.

Query APIs and their clients which support v1.3 of IS-04 or operate in a mixed-version environment MUST be tolerant to the presence of Source attributes and values which may be added at a later date. This is further detailed in the [IS-04 Upgrade Path](https://specs.amwa.tv/is-04/v1.3.1/docs/6.0._Upgrade_Path.html) document.

## Attributes

Note: JSON schemas supporting validation of all the attributes will be defined in this register.
These MAY be used in addition to the schemas, such as _source_generic.json_ and _source_audio.json_, found in the AMWA IS-04 specification.

### USB Devices
- **Name:** `usb_devices`
- **Description:** Describes the USB devices accessible to a Receiver via the USB data stream.
- **Specification:** [AMWA BCP-007-02 v1.0](https://specs.amwa.tv/bcp-007-02)
- **Applicability:** `urn:x-nmos:transport:usb`
- **Permitted Values:**
  - Since AMWA IS-04 v1.3, Array of `usb_device` objects 