Fi Packs are different groupings within the Fi App (Miniature Poodle, California, etc.) Your dog can join various Fi Packs for fun competition to see how their activity compares to other dog’s within the Fi Community. The Rank Tab shows the results of their activity between the different categories of Fi Packs. See: Rank Tab
For more information on the Fi Packs your dog can join, see: What Fi Packs Can My Dog Join?