Edited see comments below
This is from the BMC website. Cut-and-paste for reference.
BMC’s frames are made mostly using UD carbon fibre. UD stands for
unidirectional. Unlike more traditional carbon fibre products that use
a woven carbon finish (criss-crossed finish like a woven piece of
material), UD carbon is literally a piece of carbon material with the
filaments all running in one direction. BMC’s frames are made from
many different pieces of UD carbon that are woven together, similar to
what is done in textile production. Some of our frames contain around
350 individual pieces of carbon fibre that go into the mould before
heating and curing takes place. These material sheets - some very
small, some very large - are laid at different angles across each
other to achieve different results: torsional strength, compliance
areas, rigidity, and so on. The layup of these sheets is extremely
important to the end result. This is actually the art of building a
carbon fibre frame.
On our unpainted frames, you can see the result of these carbon fibre
sheets overlapping each other through the clear coat covering the
frame. We are actually very proud to be able to show this real effect
on our frames. It shows not only the true workmanship of the frame,
but also the resulting quality as the frame exits our moulds. Many
bike manufacturers prefer to paint their frames because there are
often many imperfections in the finish that need to be re-worked using
a filler before waiting for it to dry, sanding it, and then giving it
a final paint job (a bit like a panel shop). This is a cheaper way to
finish the frames, ensures you have no reject quantity, and allows you
to use less stringent production techniques.
On unpainted BMC frames, when you look 'into' the frame, you see the
actual quality of what’s on the inside – we are showing you our finish
and quality and we are very proud of that.