1) Onboard Credit is a credit that's applied to your onboard account. Everything you purchase while on the cruise (excursions/tastings/spa/nursery/gratuities/photos/drinks/popcorn/gift shop purchases) is charged via your KTTW card to your onboard account. The only OBC that has a limit it the $50 OBC that the Disney VISA gives for using it to pay for your cruise - that's only applicable to beverage purchases.
2) Category 5 & 6 are both verandah rooms. 5s will usually be on a higher deck than 6s. A, B, C, D roughly designates the actual location on the deck (A's are pretty much midship, with B, C & D progressively more forward or aft from there).