A nom that is presumed to be ephemeral is a product reviewed on this site that, in my opinion, will soon be discontinued due to lack of success. Whether its production or retail run is weeks or months long, any product that I deem a "Presumptive Ephemeral Nom" is one that you'll not be able to obtain, say, 2 years from now.
How do I know? Sometimes a company will remark that a product is for a limited time only even if they do not market it as such. Sometimes such products enjoy success and are later established as permanent ones, and sometimes not.
Aside from that, I'll be using the wisdom that I feel I've gained over a couple of decades of enjoying such products. Whether the quality is bad, the flavor of the product is not finely tuned to American tastes, or even technical reasons such as when a product is good overall but doesn't have room in market segment, or when a product is too poorly marketed to breach consumer awareness. It's usually a combination of these factors, along with my memory of what is successful and what is not that will guide me in my decision.
As for hard-to-find and regional products, I can only do my best with the information I'm able to dig up concerning its availability. Any product that's hard to come by qualifies as "Ephemeral" for the purposes of this site.
I'm not infallible, and I encourage you argue with me in a post's comments on whether any given nom should be presumed ephemeral. Also, I vow that if any products I review end up being permanent and widely available, then I will take down my post.