Software Engineering

10.2.4 
Functional interface
Data that is exchanged between functions represents a functional interface. A func-
tional interface identifies a requirement for data to be exchanged. The functional 
interface provides the initial basis for establishing a functional interface specifi-
cation. It should identify and characterize the data elements being communicated 
among the interacting software functions internal to the software product.

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CHAPTER 10 
Formulating the Functional Architecture
10.2.5 
External interface
Information that is exchanged between the software product and external systems, 
devices, users, or other software applications represents an external interface. 
External interfaces should have been specified during the software requirements 
analysis and addressed by the functional architecture to support interface design.