Phil Wood makes two headset models:
Standard, which uses external (EC) cups, and is designed for a straight 1 1/8" steerer tube. This headset is otherwise known as an EC34 type.
Tapered, which uses an internal (IS) top cup, and an external (EC) lower cup to work with a 1 1/2" fork crown. Both cups of the tapered headset are sized to fit into a 44mm headtube.