First up I will explain the difference between the carbon weaves and fined look of the rims.
On most Nextie rims you have the choice between UD, 3K and 12K weaves. UD is short for Uni-Directional, the fibers of UD weave are arranged in one direction, which is the most frequently used to make carbon fiber products. 3K means there are 3,000 filaments per "row", 12K means there are 12,000. They are decussated like a web.
The illustration is as below, it shows different appearances of the weaves, and bicycle rims in different weaves.
UD weave has the appearance of plain black. Crossed 12K weave pattern is 4 times larger than 3K weave.
There is very little difference in performance between each of the different patterns of carbon fiber. Most carbon rims are made with UD carbon. The performance of the rim is more influenced by how the carbon is cut and laid in the mold. The appearance of the rim can be changed by adding a cosmetic layer of either UD, 3K or 12K weave to give the look you are after.
However there are other elements to take into account when making a Nextie carbon rim. A Carbon fiber rim isn't just carbon fiber it is a composite material made of carbon fibers and resin that is used to bond carbon fibers together, 3K carbon fiber is decussated ((of two or more things) cross or intersect each other to form a cross). 3K carbon fibers function like a web to filter redundant resin inside of carbon fiber products. When manufacturing Nextie rims, we usually add a layer of 3K carbon fibers inside of rim channel to filter redundant resin, to balance the content rate of carbon fibers and resin material. Another advantage of 3K carbon fiber is that it protects the spoke hole from flaking when drilling it. That's why Nextie add the layer of 3K weave fibers to the inside of rims. The advantage of UD carbon fibers is strength. Nextie makes all rims with UD fibers to support.