@Hilary I got the information mainly from Jack Kruse but also other sites when I was looking to build one myself.
I think the reason is IRA is best because it contains the most frequencies that molecules in our body respond to whilst heating up minimally. I think it also penetrates deeper as the longer wavelengths get absorbed by water, though not 100%.
I would personally try to avoid getting myself too hot, the benefits of a sauna are not necessarily the heat but the photobiomodulation which is most at IRA (600-1000nm). It has been proven that these wavelengths help mitochondria produce ATP, but I think it has much more effects.
The nn stands for non native EMF. Basically most man made electronics will have negative effects on your cells as proven by research. If your are very close to the source as you could be in most portable saunas, then you will be getting a very high dose which destroys the EZ of water in your body, as far as Im aware, and this will likely do more harm than the benefits of IR.