"Current" unit in A1/A2/B1/… collections

Spanish has collections for A1/A2/B1/… and multiple units within those collections.

I find that very confusing and unintuitive.

  1. How is determined which unit is marked as “Current”?
  2. When I press the button “Play”, I often get the sad Pikachu saying there are no more cards to play. How is this supposed to be used?