Dear Lokmane,
The two different nominal capacity values allow you to separate the "nameplate" capacity from the "design" capacity. SAM uses the nameplate capacity to calculate costs. It uses the "design" capacity to size the physical system.
For example, the default physical trough system has a nameplate capacity of 100 MW and a design capacity of 111 MW. This means that the project costs will be calculated from the 100 MW value, and the solar field will be sized for a power cycle with a nominal capacity of 111 MW.
If you do not need to separate the capacity values in this way, then you could set the "Estimated gross to net conversion factor" to one and use 60 MW as both the nameplate and design capacity.
Best regards,
Paul.