Registration credit
Registration credit or R is a grade by which get credit but no actually letter grade or P/F grade.
Students apply for R credit when their internships require that they receive college "credit".
It is also used extensively in graduate school. In many language departments, students have to take something like 70% of their classes for a grade. This usually means writing a paper, so if they don't want to do that, they take the class for R credit. It creates a lot of awkwardness in 4000-level classes.
Seniors in Columbia College can petition to take a class for R credit if it is over and above the credits required for graduation.