Understanding Run Back in Burner Management System

Hello There,

While discussing it with my seniors, it just came to my mind what exactly is Run Back in Boiler operation? Up till now I understood it as the error signal for a closed control loop which is necessary to run a Boiler at different loads like 30%, 60% and 100% MCR (maximum continuous rating) which in turn decides the number of F.D. fans required. But this error or run back always is 5%, i.e., Run Back is always 5% more than PV (process value). Why is this so?

I hope my query shall receive kind concern of people on this forum.