This is a good question.
Where do you stand on a "Fox's Chocolatey Milk Chocolate Round"?
It's fully enrobed in chocolate, but its pack layout (loose in a pack, not individually wrapped), shareability and dunkability says "I am a biscuit".
It does, however, lounge in twos or threes inside a plastic tray inside its packet, rather than the Digestive/Hobnobs who simply accept that they will sit directly on top of one another, all the way down.
Is it in its own category as "fancy biscuits"?
(I'm sure some biscuit VAT man has already been through all of this, after the Jaffa Cake tax issue)