Nesting is a way of one template encompassing one or more other templates.
As it makes sense to separate out on individual templates entities for various services, applications etc. you may end up with quite a few templates all of which may need to be linked to quite a few hosts. To simplify the picture, it is possible to link some templates together, in one "nested" template.
The benefit of nesting is that then you have to link only the one template to the host and the host will inherit all entities of the linked templates automatically.
If you want to link some templates, to begin with you can take an existing template or a new one, then:
Thus, all entities of the template itself, as well as all linked templates' entities (such as items, triggers, graphs etc.) will appear in the template configuration, except for the linked template screens, which will nevertheless be inherited by hosts.
To unlink any of the linked templates, in the same form use the Unlink or Unlink and clear buttons and click on Update.
Choosing the Unlink option will simply remove the association with the other template, while not removing all its entities (items, triggers, graphs etc).
Choosing the Unlink and clear option will remove both the association with the other template and all its entities (items, triggers, graphs etc).