Not familiar with this particular implementation, but it almost certainly stands for Programmable Limit Switch. In the old days, when you wanted to have certain functions occur at certain phases on some piece of rotating machinery (like a press, say), you'd have used a cam box. A box with a bunch of adjustable cams all on the same shaft, each with a limit switch.
The next refinement was to replace the cam box with an encoder connected to a dedicated electronic monitor. The monitor had a number of output relays and you could program them electronically rather than have to cut a new cam ("Have switch 1 on from 90-185 degrees and then again from 240-270"), and was called a Programmable Limit Switch. I'm sure it's this kind of a function the "PLS" mode refers to.