I personally understand bioload solely as pound of fish per water volume .
Bacterial build up is refered to the anaerobic bacterias (fish pathogens ) that consume organic waste as their main fuel unlike the Beneficial bacteria species that thrives in high oxigen water .
So basically we aim to get more Nitrosomonas and Nitrobacter and less Pseudo/aero monas in our system .
Just my 2 cents and propably "oversimplified " belief on this complex subject .