bonus nights are nights that you have earned, not by staying at a marriott, but by either using their credit card or rollover nights from the previous year. i.e. it takes 50 nights to make gold. if you have 70 nights this year you get 20 nights rolled over to next year as "bonus nights". If you have the credit card you also have 15 additional bonus nights. if you spent $9000 on your marriott credit card you would have 3 more bonus nights. That way you would start next year, in this example, with 38 "bonus nights" that count towards your total nights stayed for your status (silver, gold, platinum) next year. This is to distinguish from how many nights you actually stayed in a marriott that year. i.e. marriott rewards would treat you like you had 38 nights plus whatever you actually stay the next year. Sounds like you have a lot of nights earned from a marriott visa card.
Okay. I think that you're probably referring to your account and the bonus nights listed there are roll-over nights after the 75 nights for platinum qualification are deducted from your stays last year.
We had 269 that were listed as bonus nights. They're only used to make a level this year if you need them, but they disappear after this year from what was carried over to this year.
It might have been something like this that kept us (Larry) in elite when he didn't travel in that particular year, but I think it's just recently become official.