Make explicit qualities and restrictions for the design
Design requirements and constraints respectively stand for non-functional requirements the final product should meet and restrictions the designer is faced with.
A good exploration of design solutions is only possible when the design challenge is clearly defined, which is typically done in the form of requirements. These can be categorized as either functional or non-functional. Functional requirements state what the product should do. Non-functional requirements state how the product should provide functionality. Design requirements belong to the second category and cover needs mainly related to product qualities and ergonomics.
Design constraints on the other hand are boundary conditions by which the design is restricted. Examples of design constraints are the available budget, the technical framework on which the product needs to run, devices on which the software will be provided, etc.
The design requirements and constraints result from confronting and balancing needs and vision of all stakeholders (users, marketing department, product and project managers, developers…). They are summarized in the design brief, which forms the input for the designer and based on which the design will be tested or evaluated afterwards.
Developing products remains mainly motivated by the need or desire to provide functionality. Functional requirements in consequence form the priorities while non-functional requirements often remain implicit and unspecified. They are however of great interest in order to differentiate and to improve the overall quality of the final product.
Writing and maintaining understandable, unambiguous and testable requirements demands a significant effort because