An expansion set of player cards only, published separately from the base game each year. Gamers who already owned the base game (which included a full set of cards for a year) could buy the new cards for the previous season, and play the teams without having to buy a base game again.
The package held a set of large perforated sheets for all the actual NFL teams and players, using revised statistical information based on the season passed (so the 1992 published set had the 1991 year stats). Some of the index cards included new rules or clarifications.
The packaging made it very clear that you needed the base game, so if you didn't know what it was about "it behooves you to gently place this package back on the shelf and purchase the complete game".