In my Carol S. Buck, 2012 Medical Coding book, the glossary definition is this:
Separate procedures: minor procedures that when done by themselves are coded as a procedure, but when performed at the same time as a major procedure are considered incidental and not coded separately.
Not sure that was plain English.
For example, 49320 Laparoscopy, abdomen, peritoneum, and omentum, diagnostic, with or without collection of specimen(s) by brushing or washing (separate procedure). If the person goes in for this procedure and has nothing else done then you can code it, because it is a separate procedure. If a person goes in for the 49320 laporoscopy and also has and unrelated surgery done, say a bunion repair, then you can code it, because the lapaoscopy is a separate procedure. But if the person has a 49320 laporoscopy and in the same surgical session that precedes a repair to the abdomen then you can't code the 49320 laporoscopy because it is not separate, but included in the repair.
This mystified me for the longest time, so I liked that you asked your question because I had the same one when I was a student.