Most whole grains are seeds like brown rice that have a hard outside hull that holds the seed. That part is removed so we can enjoy the seed much like cracking the hard white shell on a pumpkin seed to eat the green inner "fruit". We consider that inner part whole when it has all of its layers intact. A grain of brown rice has three distinct layers. The bran, the germ and the starch. The inner starch is white rice and contains barley any of the nutrients that you will get from the germ and the bran. Eating the whole seed is superior to the parts.
Some hulled grains are really seeds that only have one layer such as Jobs tears barley as opposed to pearled barley which is more like the white rice described above. I believe buckwheat groats are just hulled buckwheat and are gluten free, high in lysine and take longer to digest so you feel full longer from eating them. I would say skip the buckwheat those if you have skin sensitivity for they can sometimes make skin conditions worse.