If business process (logic) is part of the business components or means thew same thing, how come they are named as separate layers?
Basically, if you want an answer to this question you will have to ask the individuals that created your system. They are the only ones that can give you an answer because this is something that they created. Your description clearly reveals that the system was not designed in accordance with Sun's J2EE programming model.
The term "tier" when used in the context of Sun's J2EE three-tier programming model is synonymous with layer.
The term "tier" also has different definitions in computer science and construction. However, in regards to the J2EE model, the author's made the decision to use the term "tier."
The Integration tier is where the external and non-object-oriented components are. This includes relational database management systems, file-systems, web services, other information systems and programs.