"Is rock-art (esp cups and rings)...representative, communicative or aesthetic and will we ever know?"

It continues up until the present day. People are still making Rock Art, of various forms, right now. There's not many of them, and they're not working in a tradition, generally, but it is still going. The gargoyles and flourishes on ancient churches are rock art. So are the Celtic Heads.

The cup and ring motif is well known and easily recognised. Cupmarks, of various form, are found around the world. They are generally assumed to be prehistoric, but that is because they are associated with other ancient monuments. Some of the English cupmarks attributed as prehistoric might even be medieval (or later).

There are said to be three stages of response following new discoveries. Silence, ridicule and hearty acceptance. A person finding a cup and ring marked stone on the moors of North Yorkshire might expect the three stages to occur quite rapidly - in a couple of weeks. A person finding something that has not previously been seen before might expect to wait years or even decades ...