Service definitions can also be tagged in order to inform the container if they serve a specific purpose. Typically, these are picked up by a collector service that uses them for a given subsystem. As an example, if we wanted to tag the hello_world.salutation service, it would look something this:
hello_world.salutation:
class: Drupal\hello_world\HelloWorldSalutation
tags:
- {name: tag_name}
Tags can also get a priority, as we will see in some examples later in this book.