We have collected and list, for each of the layers, the responsibilities it must fulfill and problems which may arise, together with a reasoning. The architecture layers should not be mixed up with tiers. A clean layer structure of an application is a precondition for distributing the application on different tiers.
Similar considerations apply to a three-tier system.
A business process requests services from business entities. But a business entity cannot request services from a business process. A business entity has typically a permanent state, which may be changed by the execution of a business process. A business process has, once its execution is terminated, no permanent state and even no permanent lifetime. The sharing of business entities involves elaborate transaction mechanisms to prevent that parallel requests of state changing business services lead to wrong results.