Drupal 8 8.0.2
End of "defgroup ajax".
Service tags overview
Some services have tags, which are defined in the service definition. Tags are used to define a group of related services, or to specify some aspect of how the service behaves. Typically, if you tag a service, your service class must also implement a corresponding interface. Some common examples:
Creating a tag for a service does not do anything on its own, but tags can be discovered or queried in a compiler pass when the container is built, and a corresponding action can be taken. See for an example of finding tagged services.
See Services and Dependency Injection Container for information on services and the dependency injection container.