There is no specific Article 501 rule for installing horizonal seals; however, Section 110.3(B) requires listed or labled equipment to be installed per instuctions included with the listing or labeling. Seal fittings must be listed per 501.15(C)(1) AND their manufacture's instructions will indicate if they are suitable [Section 500.8(A)] for horizonal installation.
Section 501.15(C) applies to seals in both Class I, Divisions 1 and 2.