What is DU bearings? - bearing knowledge

What is DU bearings? DU in the presence of a bearing on a lot of, basically be to see your bearing brand, my next example: French SNR bearings complementary code DU said general installation in pairs; ( Pairing can use) French SNFA bearing complementary code DU said universal matching is used, which means that the bearing for matching use, face to face, back to back, series can be normal operation, and has no effect, generally 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)