Yes, it is correct to use the UNL parameter in such case.
The use of all design parameters is explained in the chapter 2B.6 Design Parameters of the International Design Codes manual which can be found in STAAD.Pro Help.
By the way, you are using a very old version of STAAD.Pro, so, I think, updating to the most recent version (build 20.07.10.64) would be a good decision.