A number on its own is still ambiguous, because it does not say when it applies. A quality attribute scenario is a fixed six-part sentence that removes the ambiguity. The source is who or what acts. The stimulus is what arrives. The environment is the state the system is in when it arrives, and this is the part that carries most of the information: normal operation, peak hour, one machine already lost, a dependency degraded. The artifact is the part of the system that is stimulated. The response is what the system must do. The response measure is the number, with its distribution point and its window. Written out: a signed-in customer submits a checkout during the busiest hour with one instance already failed, and the order service accepts or refuses within four hundred milliseconds for ninety-nine percent of submissions. The form is worth the effort because each slot is a question people forget to ask, and because two scenarios that differ only in their environment can demand completely different structures. It also makes a design reviewable: you walk a candidate design through the scenario and see whether it survives. After this Concept you can convert a stated quality target into a scenario with all six parts filled, and say which slot a vague requirement has left empty.
A quality attribute (QA) is a measurable, testable property defining how well a system performs a responsibility (e.g., performance, security, availability, modifiability), as distinct from functiona…