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I am trying to figure our if the following system requirement is a Functional requirement or a Non-Functional Requirement?

"The Stock Control system must update the stock item quantity when a stock item is sold."

I personally believe that it must be a NF requirement, due to the 'when' part.

please help to clarify.

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This is a functional requirement.

Compare it to the definition of a functional requirement from Wikipedia:

A function is described as a set of inputs, the behavior, and outputs.

Functional requirements [...] define what a system is supposed to accomplish.

The Wikipedia article describes the difference between functional and non-functional requirements as well.

Generally, functional requirements are expressed in the form "system must do ", while non-functional requirements are "system shall be "

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In addition to the wikipedia definition of NFR, I would say a NFR describes how the system (or a specific feature) should behave; while a functional requirement describes the behaviour itself.

Other thing you could keep in mind with these definitions is "Is this statment applicable throughout the application?". A requirement that says "The user should be able to sign in" is the behaviour itself - it does not extend other definitions. While "the user should perform any activity in 15 clicks, maximum" is a extension of behaviour that is applicable to other behaviours. The NFR Framework makes this extension characteristic explicity.

Wikipedia has a list of NFRs that may make more clear what is a NFR.

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It is a Functional Requirement, Clue one: "Must"

Clue two: part of correct working of the system, (if ignored, you will be selling products you dont have in the inventory)

clue three: your other systems like auto re-stock etc wont work without this.

this is so far deep in the functional requirement section, that it is even not visible from the boundary of Functional-Nonfunctional reqs.

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