Involved in the type II fatty acid elongation cycle. Catalyzes the elongation of a wide range of acyl-ACP by the addition of two carbons from malonyl-ACP to an acyl acceptor. Can also use unsaturated fatty acids. Catalyzes a key reaction in unsaturated fatty acid (UFA) synthesis, the elongation of the cis-3-decenoyl-ACP produced by FabA