Refundable OBC is real money, given to Azamara by your TA to credit to your onboard account. It's your money, that's why it's refundable to you if not spent onboard.
Nonrefundable OBC is funny money credited to your account by Azamara to spend on marked up stuff (excursions, specialty restaurants, shop items, spa , etc.) sold on the ship. So if you choose to put $nnn into the crew fund, that refundable OBC is real money coming out of your pocket, so dollar-for-dollar or pound-for-pound or whatever it goes at full value to the fund, as if it came straight out of your wallet. A dollar of nonrefundable OBC is not worth a real dollar. It's only worth a dollar of overpriced ship's stuff. If Azamara ever let you give gratuities to a crew member in nonrefundable OBC, I am puzzled as to why.