The idea is that, as long as you're treating the product as a "black blox" (not peeking inside to see how it works internally), you're in the realm of requirements and not design.
This distinction has a great deal of initial intuitive appeal but leads to absurd conclusions with contradictions in terms. For example:
P1. User interface designers produce specifications that describe only external aspects of the product.Requirements are about specifying the problems the product should solve and avoid. Nothing more, nothing less.
P2. If a specification describes only the external aspects of the product, it does not contain design.
C. Therefore, the specifications that user interface designers produce do not contain design.