For example, cleanroom has classification that implies some amount of dust particles/impurities in the air. The technicians don't talk about the number of dust particles allowed, they just consider class 5 suitable for some applications whereas others might need class 4.

Class 4 cleanroom makes sense, as does "Ultra pure" as long as industry sees it as a standard.

Another example: UHD makes as little sense as FHD in terms of display resolution. I mean, Full must mean it's at maximum possible amount of high definition? Well we just know it stands for 1920x1080, and we know UHD or 4K is larger.