The core type decides how the arc behaves and where the wire is used. Rutile wires, marked RC, give a smooth weld face, slag that lifts easily, and work in all positions. That is the choice for steel structures and for vertical-up welding. Basic wires, marked BC, have the best impact toughness at low temperatures and the lowest hydrogen, but they take a practised hand. Metal-cored wires, marked MC, leave practically no slag and have the highest deposition rate, which suits mechanised welding of long runs.
The seamless construction matters in practice. A seamless wire does not pick up moisture from the air, so it does not have to be dried once the packaging is opened and it can be fed through long liners without concern. Wires rolled from strip have to be stored somewhere dry and used within a sensible time.
Self-shielded wires work without gas, so they do well outdoors in wind. They do, however, require a polarity change on the machine and they produce more fume, so you need working extraction.
The usual diameters are 1.2 and 1.6 mm, and 1.2 mm is used for out of position work. The working current for 1.2 mm is normally 150 to 300 A, for 1.6 mm from 200 to 400 A.
With flux-cored wires use knurled rollers and set the feed roll pressure lower than with solid wire. Too much pressure crushes the core and makes the feed uneven.