Depending on where you live depends on the answer you will receive. In MD State Jobs it is based on years of service and experience. The higher the I, II, III the higher the grade you are and more $ you receive.
The career ladder at my hospital is that all new grads start at LPN 1 or RN 1, after 6 months you can be classified as LPN II or RN II. That is the highest LPNs can go, but a RN/BSN can get RN III after x amount of time and by doing some sort of project. RN IV is someone with MSN. Each level means a higher pay scale.