This rate shows by default the fixed rate as calculated by the BGM model that will automatically result in a zero NPV. That is, it displays the breakeven rate.
It is displayed in a mid format.
If you then define your own fixed rate (via the Market Rate field) SD takes that edited rate as the fixed rate input for the BGM model (and displays it in the BGM Rate field) and then calculates the resultant NPV.
For more information on the BGM model see Pricing an Instrument Using the BGM Model.