Free catch is never sterile. Not even in humans. The only way to get a sterile sample is needle into the bladder. It is a "clean catch", but not sterile.
So then, what would a 'dirty catch" be? It's when an animal is caged and you put a pan under to catch the urine. You have the pan there for several hours and it can contain food, poo, fur, whatever that animal can drop into the pan.
(I'm ASCP certified Medical Technologist. I do labwork)