U is for umbra, a category of secrecy (compartment), now out of service, that allowed people with UMBRA clearance to have access to highly sensitive communications intelligence.
An umbra is the blackest part of a shadow cast by an opaque object, especially the cone-shaped shadow cast by earth or the moon during an eclipse. This dark shadow is obviously an apt metaphor for the secrecy, the 'dark sites,' and the covert activities that U.S. national security agencies have engaged in at an ever accelerating pace since 9/11/01. But there's another shadow side to America's 21st century security state: it has grown so quickly and become so large and unwieldy that no one, including those inside the NSA, actually knows how much money we spend on it, how many people are employed within it, or which programs are doing what work and how much of that work is duplicated. As for oversight, over 100 congressional committees claim at least some oversight on NSA activities.
5 applications of color, with blind emboss
Kochi Kozo paper