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I have been in a muddle about understanding exactly what differentiates functional specifications and non-functional specifications from one another. 

I understand that functional specs are concerned with how a system should behave, whereas non-functional specs are concerned with how a system should operate.

My understanding becomes muddled when determining what models should be used to specify non-functional requirements compared to functional ones.

For example, use cases or user stories are solid modelling techniques for specifying functional requirements, but when it comes to data modelling I'm unsure whether this falls into non-functional requirements or not.

If data modelling does not fit into non-functional requirements, how then do BA's specify these requirements?