The 'CT' is 'Cubic Tech'
The '2K' is the amount of spectra
The '.08' is the thickness of the mylar membranes on either side.
Sometimes you see additional letters in there like 'HB' which stands for 'High Bias' which means it has spectra layered at +/- 45 degrees in addition to the regular stands at 0 & 90 degrees.
CT2K.08 is the normal 0.74oz/yd stuff. The lighter cuben's being used use the same mylar layers but less spectra. 0.33oz is CT0.3K.08, 0.48oz cuben is CT0.6K.08, 0.51oz cuben is CT1K.08.
The heaver 1.5oz cuben used in some applications (ie. packs, some tent floors etc) is CT5K.18. Notice the membrane is over twice as thick. This was the lightest version of cuben that used the thicker membrane, but now there is a 1.2oz cuben out that uses the .18 membrane but less cuben...probably around 3K I'd guess. This seems like great stuff for groundsheets, tent floors etc where abrasion is the main concern.