Concept: Business Subsystem (or Business Service Interface)
Business subsystems and business service interfaces are equivalent elements for aggregating business services that are assigned to business functional areas.
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Main Description

Once business services are allocated to a functional area, it is helpful to group them to reflect affinity between functions.  This grouping can facilitate subsequent mapping into the IT domain.

Two common aggregation approaches are (1) business subsystems and (2) business service interfaces.

A business subsystem (or, simply, "subsystem") is a conceptual aggregation of cohesive functionality within a business functional area.  It is "owned" by the functional area, as a sub-component.  The UML representation would be a stereotyped component which exposes an interface that aggregates the appropriate business services. 

An alternative approach is to aggregate the related business functions using business service interfaces that are exposed by the functional area, itself.  If one wishes to show further internals of the functional area, it is straightforward to create the internal business subsystems and delegate behavior from the functional area's service point to a service point on the business subsystem.

The aggregational approach that is used does not have significant impact on subsequent activities related to mapping between business functions and IT.  In one case, a conceptual IT subsystem will be mapped to a business subsystem.  In another case, a conceptual IT subsystem will be mapped to a business service interface.  In either approach, we bridge between business (as defined by functional areas) and IT (as realized by service components).

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