Unums are an odd duck, but the related posits from the same person make a lot of sense. They extend the range of floating point numbers by slowly tapering the relative accuracy the further you get away from 1. I find the paper[0] very readable, but if you just want some quick examples of the numbers, I wrote a blog post[1] some time ago which you may find helpful.
[0] http://www.johngustafson.net/pdfs/BeatingFloatingPoint.pdf [1] http://nhaehnle.blogspot.de/2017/09/posits.html