Its the firmware revision on the card. Each card has a special area where code snippets can be written and hooked into the cards main program. They are normally used for fixing bugs in the original program but can also be used for other purposes.
A16 is an image that has minimal bug fixes (just a couple I think). The latest image for a Rom10 is A82 which, if written to your card, will lock it instantly requiring you to use an unlooper.