If a course has a grade mode of Satisfactory/Unsatisfactory no one in the class gets a standard letter grade; everyone earns an S or U. When you elect to take an otherwise graded class pass/fail, the professor still assigns you a grade from A-F but the Registrar's system converts it to pass or fail. Juniors and seniors are permitted to elect to take one pass/fail class per semester. Having an S/U class on your schedule has no impact on your ability to take a class pass/fail.
The Mendoza Office of Undergraduate Studies is ready to assist our undergrads!