What does functional and not functional mean

Functional, non-functional requirements

In requirements engineering, a distinction is made between functional and non-functional requirements.
Functional requirements describe the desired functionalities (what should the system do / be able to do) of a system or product, its data or behavior.
Non-functional requirements are requirements for the "quality" in which the required functionality is to be provided.

Quality in the aforementioned sense means for example
  • how the functionality should be carried out (e.g. response time)
  • Conditions under which the functionality is executed (e.g. 7x24 hours)
The distinction between functional and non-functional requirements is not essential - it is essential that both types are fully taken into account in the project.

Usually it turns out that the client mainly thinks of the functional requirements. Only think of the non-functional requirements where they have had bad experiences in the past (e.g. insufficient availability, poor quality, etc.)
However, reasons can be defined why the distinction between functional and non-functional requirements can be essential:

Functional requirements differ from product to product; i.e. if a bank builds different applications, the functions in these applications will very likely differ. Non-functional requirements, on the other hand, can be specified / suggested or even standardized for many products from the same company (e.g. uniform decision-making and thus "similar" usability; specifications of operating modes which are supported by the organization, etc.)

A second reason is that the non-functional requirements often affect the entire architecture, the entire design of the product, while the functional requirements usually only have "local" effects; i.e. if the non-functional requirements are "screwed on", this normally has far more far-reaching consequences than with the purely functional requirements.

A third reason is that non-functional requirements are often only / only recognized as requirements if bad experiences have been made with this topic in the past. Or would you define a requirement for the performance of your new PC if you had never had performance problems before?
From a project perspective, however, it can be said that - with the exception of the characteristics defined in the previous paragraph - the distinction between functional and non-functional requirements is not essential - it is only important that both types are taken into account!

You can build a system that perfectly covers and fulfills all functional requirements; if the non-functional requirements (e.g. usability, response time behavior, etc.) are not correctly taken into account, the system will not be accepted by the user / customer.