How does one 'track' a subplot?

Basically it means to pull out each of the beats of a subplot and see if/how they play out cumulatively from Beginning, Middle and End. Is there a narrative flow for each subplot? Is there a thematic coherence for each subplot? While there, it's a good idea to ask a fundamental question: What is the function of this subplot, why does it exist? My principle on this front is: Subplot = Relationship. Most often every primary character in a story is going to have their own subplot in relation to the Protagonist. So a good way to track subplots is focus on the relationships of each character with the Protagonist.