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Application tier
This tier normally deals with business logic and any computation that may be required. Some persistence is required at this level as the computations might need older references. An example of an application tier would be JBOSS, TomCat, IIS Application Pools, and so on.
You might have already realized that the IIS application in Windows collapses the web and application tier.
Some example of functions performed by the application tier could be logging in to the application, encoding videos, processing payments, and so on.
It is now almost standard that the application layer's input is the API layer, so the web is decoupled from this layer.