Note: If you upload a print-ready file you should submit one single PDF document containing all pages - both front and back for a 2-sided card and both outer and inner pages for a folded card.
For standard 2-sided cards, the first page of the design indicates the front and the second page is the back.
For folded cards, on the vertical orientation page, 1 in the editor represents the front and the backside.
Page 2 shows inside pages of the folded card:
The same principle applies to pages 1 and 2 when designing folded cards with horizontal orientation: