It was so-called because it was about 80% of PL/I. Written in PL/I and bootstrapped via the PL/I Optimizing compiler, it was an alternative to PL/S for system programming, compiling initially to an intermediate machine-independent language with symbolic registers and machine-like operations.
This project was called such due to its similarity to PL/I. It was written in PL/I and bootstrapped using the PL/I Optimizing compiler, serving as an alternative to PL/S for system programming, producing an initial intermediate language with symbolic representations of machine operations.