[ATDrake]

Card on file, which you can change at any time. If perhaps someone stole your wallet and you had to cancel all your old cards, you can set a new one as default and re-download your books, though the old files you previously downloaded will merrily still open with your old CC details forever after, without having to "re-authorize" anything with B&N's central server.

Gift card payment doesn't matter. Your CC won't be charged unless your GC runs out, but the book will still be DRM-ed to the Name/CC# entered in your account. I think I've read some international people on this forum have problems getting B&N books with only GC and no CC, because the system won't let them, even though they have the money, but I could be misremembering.

Basically, the B&N scheme works just like a username/password combo where they hope you value that info enough not to pass it around. Because Nook devices and apps do accept multiple different combos and even stores the hash so that if you trust someone, you can basically do a book-swapping back-and-forth with their files on your reader and vice versa.