Actually, there are six types in this particular ASTM A513 Specification. Types 5 and 6 are the only two that are DOM. ASTM A513 Type 1 is Hot Rolled Electric-Resistance Welded (ERW) Carbon Steel Mechanical Tubing. ASTM A513 Type 2 is Cold Rolled Electric-Resistance Welded (ERW) Carbon Steel Mechanical Tubing. ASTM A513 Type 3 is "Sink Drawn" Hot-rolled Steel. ASTM A513 Type 4 is "Sink Drawn" Cold-rolled Steel. ASTM A513 Type 5 is Drawn Over Mandrel (DOM). ASTM A513 Type 6 is Drawn Over Mandrel Special Smooth ID (SSID DOM).
In the DOM process, from what I understand, a mandrel is placed inside a generic tube size and the tube is cold drawn to size by being pulled through a die, which as GRoberts indicated, causes the weld seam to virtually disappear. The die controls the outside diameter, and the mandrel controls the inside diameter. The tube is stress relieved, annealed, pickled, and the weld retested. The end result is a product which is said to be superior in finish and wall uniformity to seamless tubing.