Friend Credit

From Wikitroid, the Metroid wiki

A Friend Credit is a type of Credit. It is green. You can obtain one by having someone on your Friend Roster send you a Friend Voucher. Thus you can practically get unlimited Friend Credits because they can send you as many Friend Vouchers as possible, and then start their game over. These are needed to unlock extras in Metroid Prime 3: Corruption, such as a majority of the Concept Gallery and everything in the Bonus Gallery. 15 are needed to unlock everything in one save file.