Exchange 2013 passive DB backup

How does netbackup choose the passive copy of the database to backup when there are multiple passive copies. I have 1 active db, 1 passive, and 1 lagged copy. I want NB to always choose passive copy and not the lagged copy. I haven't found any documentation that states how it behaves when a lagged copy is present (other than links about log truncation). I know I can set a server preference list, but I was curious as to what the default process is.

Thanks.