What is DU bearings?

What is DU bearings? DU in the middle of the bearing on the present a lot of, basically be to see your bearing brand, my next example: French SNR bearings complementary code DU said gm plant in pairs; （ You can match use) French SNFA bearing universal matching using complementary code DU said, meaning is bearing for matching use, face to face, back to back, series are able to function properly, and no effect, ordinary this symbol will appear on the angular contact ball bearing; The Swedish SKF bearing U ( DU) - - Contour grinding of angular contact ball bearing NSK NSK bearings D or DU expressed deep groove ball bearings, angular contact ball bearing a non-contact rubber dust cover, common type is: 6205 5205 DU DU, DDU said: deep groove ball bearings, angular contact ball bearing, double non-contact rubber dust cover, common models for: 6205 DDU 5205 DDU Germany bearing DU said two sets of general portfolio bearing ( The same as the first two explain the meaning)