Dear Javier,
The design DNI is different from the hourly DNI values in the weather file. The design DNI is used to calculate the nameplate capacities. For example, a system with a 100 MW nameplate capacity and design DNI of 950 W/m2 means that the solar field and power block will generate 100 MW during times when the DNI is 950 W/m2 (and when other design conditions are met).
The weather files has DNI, ambient temperature, wind speed, and other hourly data that SAM uses during the simulation to calculate the system's electrical output in each hour of the year based on the solar resource and meteorological conditions.
These videos about SAM's CSP models explain more about how they work:
sam.nrel.gov/concentrating-solar-power/csp-videos.html
Best regards,
Paul.