s_CO2 power cycle part load performance

Dear SAM developers,

I am trying to understand better the s_CO2 off-design performance model in SAM 2018.11.11. The specific questions are:

1. Is the off-design performance calculated based on the design point we specify?
If so, to specify a design point, the input parameters are : design point inlet and outlet HTF temperature, power cycle gross output, cycle thermal efficiency, ambient temperature, approach temperatures, compressor and turbine isentropic efficiencies. Is that correct?

2. What are the off-design parameters considered in the calculation of off-design power cycle performance (e.g. ambient temperature, mass flow rate turn down, turbine inlet temperature)?

3. What is the detailed off-design model? Is there a relation like cycle efficiency fraction=f(mass flow rate fraction)?
If the model is too complicated to show here, pointing me to a reference or open source code would be very helpful.

4. When I click the button "calculate design point and off-design performance", SAM optimizes the cycle efficiency for different scenarios. Could you explain a little more about what are the variables and objective in this optimization problem?

Thanks for the help!
Xinyue Peng