That sentence sounds misleading. It's not that that all "must be unique", it's that a set contains only one copy of a value. I know... definition of "unique", but "must be unique" implies it's the users responsibility, or some side effect will happen, where really, distinct values are part of the definition.

How about: "Sets are collections of unique binary values, such as strings. Sets can be used either on their own or embedded in a map."