Noumenon and "mind only" according to wikipedia anyway:


  The noumenon (/ˈnɒuːmɨnɒn/) is a posited object or event that is known
  (if at all) without the use of the senses.


I know that the term is used in Buddhism, at least after its sinification. Often noumenon seems to mean reality itself, the buddha nature.

Avoiding that last gloss, may I ask if the "noumenon" is "mind"?

And whether extinction, be that impermanence, nirvana or final nirvana, is noumenally real?
I think that this might be the result of a translation problem. The concept of a noumenon in western philosophy is very foreign to Buddhism I think. In western philosophy a noumenon is the opposite of a phenomenon. A phenomenon is something that is experienced, whereas a noumenon is something that is somehow outside of experience all together but somehow exists anyways.

Most forms of Buddhism don't settle well with the idea of any kind of reality having some sort of existence that is independent like that because it falls into the error of essentialism, or at least it appears to.

I think you may be coming across this word in an off translation or something relying on such a translation, because I suspect a translator may be trying to translate the concept of 體用 (tǐ yòng, usually now translated as essence and function) as noumena-phenomena, but I don't think this is a very good comparison at all. Definately the term 體 does not mean noumena, but something more like the nature of something.