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Application tier contains 2 things:
1) Applications
2) Cognos BI Server
Applications are Cognos connection, Administration, Report studio, Query studio, other Studios, Event studio and more scalable things.
Cognos BI server includes 2 things which are , Content manager and Dispatcher. ( Content manager is actually a service in dispatcher which talks with content store , so BI server is actually 50% dispatcher and 50% Content manager).
Adding to this, there is also another term coined in docs, which is, Report Server.
Which i think is used to facilitate 2 services (Report service and Batch report service), and a BIB executable process.