Functional design is the kind of design that is agreed between software architects and business customers. It describes what the system has to do in a way that both parties can understand and agree. It might describe user interface requirements, it might discuss database access, it might discuss web services or other forms of communication, it might discuss service level agreements or clustering and redundancy. The point is that it should be detailled enough to become the agreed definition of what the system will do.
Technical design, on the other hand, is the kind of design that is agreed between software architects and software developers. It describes how the system will be built to meet the functional design. It contains detail and terminology not appropriate to business customers but needed by developers. It might describe screen layouts, it might describe database table and column names, it might discuss WSDL or communication protocols and file formats, it might discuss server and operating system versions and dependencies. The point is that it should be detailled enough to enable unit tests, code, configurations, and technical documentation work to begin.
Frank Carver
Source: https://coderanch.com/t/100106/patterns/Functional-Design-Technical-Design
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