Where labels and selectors are used for grouping and selecting sets of Kubernetes resources, Annotations provide a means of adding resource-specific metadata that can be accessed by either Kubernetes or in the containers it runs.
As you just saw, kubectl apply automatically applies an annotation to track the last applied configuration state of a resource when it is invoked. In the last chapter, you might have noticed the annotation that the deployment controllers used to track revision, deployment.kubernetes.io/revision, and we spoke of the kubernetes.io/change-cause annotation that was used by kubectl to display the change history of deployment rollouts.
Annotations can be simple values or complex blocks (as in the case of kubectl.kubernetes.io/last-applied-configuration). The examples so far are Kubernetes tools using annotations to share information, although annotations are also used to share information in a container for an application to use.
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