rk is to ensure "compliance,", i.e., see whether or not you're taking a prescribed ergocalciferol. Beyond this, it has no usefulness.
25-OH-vitamin D3, or cholecalciferol, is both the blood measure and the supplement you need. This is the one that packs all the punch. Keep in mind also that it is the oil-based gelcap you want, with more consistent and efficient absorption. Tablets usually barely work at all, even if it contains cholecalciferol. Most people who take calcium tablets with D, or multivitamin with D, not only are getting a powdered form of D, but also in trivial doses. It's the pure vitamin D3, cholecalciferol, in gelcap form you want if you desire all the spectacular benefits of vitamin D.