When you divide the number of spam complaints by the number of emails delivered, you have the complaint rate. It is necessary to state that mailbox providers only send back a subset of the individual complaints via the FBL instead of the entire set. Each ESP tends to calculate its complaint rate differently.
This is the rate depicting how often your subscribers register their displeasure with the mailbox providers for getting your emails. Mailbox providers estimate the complaint rates by dividing the number of complaints by the number of messages sent to the subscribers’ inboxes.