# Annotations
Several components of the specification, like [Image Manifests](manifest.md) and [Descriptors](descriptor.md), feature an optional annotations property, whose format is common and defined in this section.

This property contains arbitrary metadata.

## Rules

Annotations MUST be a key-value map where both the key and value MUST be strings.
While the value MUST be present, it MAY be an empty string.
Keys MUST be unique within this map, and best practice is to namespace the keys.
Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by other specifications and extensions.
If there are no annotations then this property MUST either be absent or be an empty map.
Consumers MUST NOT generate an error if they encounter an unknown annotation key.

## Pre-Defined Annotation Keys

This specification defines the following annotation keys, intended for but not limited to manifest list and image manifest authors:
* **org.opencontainers.created** date on which the image was built (string, date-time as defined by [RFC 3339](https://tools.ietf.org/html/rfc3339#section-5.6)).
* **org.opencontainers.authors** contact details of the people or organization responsible for the image (freeform string)
* **org.opencontainers.homepage** URL to find more information on the image (string, a URL with scheme HTTP or HTTPS)
* **org.opencontainers.documentation** URL to get documentation on the image (string, a URL with scheme HTTP or HTTPS)
* **org.opencontainers.source** URL to get source code for the binary files in the image (string, a URL with scheme HTTP or HTTPS)