6312 indicates the type of the bearing. DU is the bearing manufacturer's NSK designation. The specific meaning is the single-side contact rubber dust cover.
One side: Only one side is covered with a dust cover, the other side is open, you can see the bearing ball and cage
Contact type: The dust cover has contact with the inner and outer rings, and the sealing performance is relatively good. However, under the high speed working conditions, the heat generation is relatively large.
Rubber: The material is made of nitrile rubber outside and has a steel core inside. The color should be red