We proof "top-to-top" (or "head-to-head"), this means that front and back of your proof will be printed so that the top of both pages share the same edge.
You would flip the proof like the page of a magazine to read the reverse side – on the horizontal axis. The illustrations below show how the front and back of your card will be orientated based on the Online PDF proof.
Landscape file
(long edge across the top)
Portrait file
(short edge across the top)