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Well, in my B&S assingment there is a DB interface which I have to implement using a class called Data.
DB declares some methods like get(recNo), search(...), delete(recNo), ... all those tasks a usually the concern of a DAO.
In a 3 tier architecture you should now create a layer (the service or business layer) on top of the DAO (DB, Data, ...) which, in very simple cases just delegates to the DAO. This service usually combines the primitive methods of DAOs to implement the business needs. Checking if a record is available before booking it could be such a task.