I know a subplot when I see one. Subplots are secondary plotlines involving supporting characters. Sometimes they link with the main character's storyline, sometimes they don't.
As I have my editorial cap on at the moment--reading my printed out WIP ten or twenty pages at a time, marking it up, making notes, etc.--I'm taking note of what is a subplot and what is not. I'm also seeing that I might need to eliminate some as I go along.
In Plot & Structure by James Scott Bell, Bell says subplots, too, must be written with a character, their desires and a conflict in mind. He didn't mention how much space a subplot should take up or how many there could be in a roughly 90,000 word novel.
Anybody have any ideas?