Involved in the assembly of the O-repeating unit during O-antigen biosynthesis. N-acetyl-alpha-D-galactosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-N-acetyl-alpha-D-galactosamine = alpha-D-GalNAc-(1->3)-alpha-D-GalNAc-di-trans,octa-cis-undecaprenyl diphosphate + H(+) + UDP Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. O86:H2 and O86:B7 subtypes share the same O unit, but the O units are polymerized from different terminal sugars in different glycosidic linkages. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.