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What's the difference between a functional specification and a function model? Don't they represent the same thing - what the product will do?

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A functional specification says WHAT a system or function should do. When you write a functional specification document you describe NEEDS without going into details of HOW you will achieve that.

A function model is a functional specification but now you already go into details about the HOW you will implement the NEEDS because you need to build a model. Also, the language in the function model is more restrictive and structured than what you might have available in the general functional specification concept.

To give an example, I could write a functional specification document to describe what a process should do with plain English, or I could model the same process with BPMN (Business Process Model and Notation) in a structured manner using standard symbols and conventions.

In other words, a function model is a functional specification, but a functional specification isn't necessarily a function model.

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  • Thanks for your help, just one question, in a functional specification how do you decompose a requirement without specifying how? Jul 4 at 8:17
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    It depends where you draw the line between what and how. This allows you various depths of decomposition. But just to be clear, there isn't a clear line of demarcation between the two. It's usually a matter of "fixing down the details". If you have fixed something down, then most likely your decomposition ended up with a solution that satisfies the need or resolves the requirement. If you still haven't fixed down a solution, then you most likely need to decompose things into its components, look at them individually, reach a solution for each, and so on.
    – Bogdan
    Jul 4 at 19:02

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