Direct current offers two hookups, and the arc physics follow the plug. DCEP — electrode positive, "reverse polarity" — concentrates more heat toward the electrode side and drives the deep penetration that rods like E6010 exploit for root passes. DCEN — electrode negative, "straight polarity" — shifts heat into the work: stick rods melt off faster and shallower, and it is the standard polarity for TIG on steel because it keeps the tungsten cool. Alternating current has no terminal to choose; the roles swap every half-cycle at line frequency, making the arc a 50/50 blend of both behaviors that extinguishes and restrikes at every zero crossing.
For stick welding the polarity question is really an electrode question: under AWS A5.1 the final classification digit designates the coating and the current it runs on. E6010’s cellulosic coating is DCEP-only and will not maintain an arc through AC’s zero crossings; E6011 is its AC-capable twin; E6013 and E7018 run on either. Manufacturer amperage tables are published per polarity — the rod amperage chart lists each classification with the polarity its ranges were published at, because the rod box, not the welder’s preference, decides.
AC earns its keep in two places. Arc blow — a DC arc pushed off course as the steady welding current magnetizes the workpiece — largely disappears when the current reverses every half-cycle, which is why switching to AC with an AC-rated rod is the textbook remedy. And TIG-welding aluminum practically requires AC: the electrode-positive half-cycles cathodically scrub the refractory oxide off the puddle while the electrode-negative half-cycles put the heat into the base metal.