SubID is added with different parameter, few ad network use parameter like this.
Here you can see I set subid Is FB because I am promoting my aff link on Facebook.
In second link I am using subid as blogfooter, because I am promoting my aff link on my blog with footer banner.
So subid are for traffic tracking, which help you to analys which traffic is converting more(lead or sell). If you don’t use it then it’s fine, but when you promote aff link on different placements then subid help you to track conversation means from which aff link you’re getting sell/lead.