Use of different rates

My projects cross several years and each new year brings a new resource rate. I found I can assign the new rate on the resource tab and it is controlled by the start / end dates at the resource level. BUT the resource level only has one field for price/unit.

Resource tab options;

I had expected a price/unit 2, price/unit 3 etc. like on the resource tab.  How are the price/units on the activity resources linked?  In the example below, I typed them in.

Activity lines