Hi!
I've seen "pl" used before "marker" to indicate you should place a marker at that point in the pattern. And I've seen it used in books and patterns where it's not defined anywhere so you're left to guess or figure it out.
Maybe the writer considered it self-explanatory, but I had to take a bit of thought and time to figure it out! So much for MY-self-explanatory!
Happy Knitting,
Ruthie