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Working while employed at large companies, I was at times confused as to what was considered to be a requirement and how a specification differs from it. Upon reading more about it, I have discovered the absense of a precise definition of a Requirement. All explanations are vague. Please, clarify from the practical experience.

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  • $\begingroup$ Wasn't this question asked recently? $\endgroup$ – Fred Oct 5 at 7:16
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Requirement and Specification are standard English words. They don't mean anything different from the dictionary definitions.

A requirement is "something you need, or something you must have." A specification is "how to do something" or "how to make something."

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  • $\begingroup$ And, a design specification per the question (rather than a process or build spec as you alluded to) is often a document including various requirements and some ideal “want to have details” that when all put together define the whole product. $\endgroup$ – Jonathan R Swift Sep 25 at 20:50
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TL;DR: (if I really had to) I would use "design specification" as a superset of "requirements"

My experience is that:

  • Requirement: tends to be used more commonly in the design phase of a product, and also imply a "absolute minimum".

E.g.: When people are on a drawing board and try to decide what the requirement for a new chair, they might say that a requirement is that it may be used for a person which is 100[kg], and the packaging dimension should not exceed 100x500x500[mm].

These two are the minimum requirements. Of course, if the chair can be approved for a person being 120[kg] and the packaging dimension of 80x450x450[mm], then the requirements are still met.

  • Specifications

The specification usually refers to (and you come across it more often when talking about) the properties of the finished product.

e.g. in the case of the chair previously mentioned, the specification of th finished product, will state that it is approved for a person of 120[kg] and its packaging comes into 80x450x450[mm]. So the original "design requirements" are now irrelevant.

  • Design specifications

Obviously this is the difficult one. The design specifications have to me a near identical meaning with requirement. "

The only difference I can perceive, is that the requirements are the CORE properties around which the design has to revolve around and they are closest to "being written in stone". On the other hand, the design specification usually refers to "requirements" and other not so essential parameters of the design. So, to my mind "design specification" is a superset of "requirements"

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  • $\begingroup$ Why do you place units within square brackets 120[kg], why not just 120 kg? I've never seen done before. $\endgroup$ – Fred Oct 5 at 9:11
  • $\begingroup$ That's what I've seen in most Journals as a widely accepted way to denote that the following quantity is units (not some sort of variable) . E.g. A could be area (a variable) or Amps. $\endgroup$ – NMech Oct 5 at 9:14
  • $\begingroup$ Out of interest, could you please give me the name of one such journal so I could see. $\endgroup$ – Fred Oct 5 at 9:21
  • $\begingroup$ see this question... I guess this officially classifies me as old :-) $\endgroup$ – NMech Oct 5 at 9:37
  • $\begingroup$ I'm actually old!! I've never seen it when describing quantities in general usage: books, journals, technical papers etc. From your example I see it is used when the units are included in equations. It makes sense now. Thanks. $\endgroup$ – Fred Oct 5 at 9:51

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