Hi,
OBC as the name suggests can be used for most spending ON board. Specialty dining, beverage packages, shops etc.
What is special to OBC, and a bit counterintuitive, is that it can be used even before embarcation but for excursions only. Not sure if that applies to all OBC or only to special indicated one. The non refundable OBC cannot be used fir extra tips.
cheers
MouseBerry