Every offer an OTA sells has both a position and a rank.
The position of an offer refers to the position of the itinerary relative to other itineraries on the results page. Position 1 means the top of the results page, position 2 is the next one down, and so on.
The rank of an offer, on the other hand, is the rank of the ticket compared to other tickets on the same itinerary. Rank 1 means the book button; rank 2 is the next cheapest price on that itinerary; and so on.
The screenshot below illustrates the difference between the terms "rank" and "position.".