Hi,
As you know, SAM has several different models and many input variables, so when you ask a question like this, please let us know the name of the performance and/or financial model you are working with and where you are seeing the variable.
The ud_T_amb_ind_od is a variable that appears on the Power Cycle input page's User Defined Power Cycle option for the Molten Salt Power Tower model in the part-load or "off design" table's Ambient Temp column:
Best regards,
Paul.