The role description 'that property which...' is intended to express a condition that is necessary and sufficient for satisfying the concept of redness.
The proposal is intended as a functional or topic neutral analysis of the concept of redness. The role description “that property which …” is intended not only to fix the referent of the concept, but also to express a condition that is necessary and sufficient for satisfying it. Thus, if the proposal is correct, then all it takes for a property to be redness is for it to fill the redness role. (McLaughlin 2003: 479)