Rate is a price you assign to an instance to make sure it delivers in the order you want in your waterfall. The rate should be set equal to the instance's intended eCPM, but the rate does not define the eCPM.

Rate behaves as an eCPM override. When you set a rate, the instance will be sorted based on the rate you've assigned, instead of the eCPM. If you have bidding active, the winning bid will be sorted in the waterfall according to the eCPM OR the rate of the other instances in the waterfall. 

Rates are overridden, according to hierarchy. Rates can be assigned on the instance, group, and country level and will behave according to hierarchy. This means that rates on the country level will override rates on the group level, and rates on the group level will override rates on the instance level.  Your waterfall will deliver according to the rates you've set on the instance, group and country level.

Sort by CPM changes instantly when a rate is updated.