What does UD, 3K, 12K carbon mean and how do they differ?
UD, 3K and 12K specify the carbon weave pattern. 3K means there are 3,000 filaments per "tow", 12K means there are 12,000 and UD means unidirectional (no pattern):
The construction of bike parts is always UD, only the top layer when naked can be specified to these different finish types. Also there is usually an option to choose between matte or glossy. Matte results in more stealthy look, glossy will make the pattern pop:
When the frame is painted it's UD underneath (usually branded frames are all painted).
As for physical properties, UD is the strongest, then there is 12K and last 3K. The finish is only the top-layer so the differences are only cosmetic.
You can also encounter a 18K weave (usually on road rims) which looks like this (glossy/matte). Its similar to 12K (but a bit bigger) and you can also see how a seam looks like:
12K is well recognisable from distance, 3K and UD you can differentiate when standing closer and they also usually look better up close than 12K (there are more visible irregularities in 12K weave than 3K up close). This is how 3K looks up close: