from online manual:
https://manuals.ootpdevelopments.com...istorical_game
Player development: You have two options here: "Recalculate player ratings before each season" means players will not develop by the game's normal rules, and instead will get adjusted each year depending on their database career settings, approximately reflecting their real level of performance that year. They will also retire in the same year they actually retired from pro hockey (note that they may have played many pro seasons after their final real NHL season.) "Use FHM development and retirement engine" will use the same development system as the regular game.
Recalculation mode is geared towards a more accurate replication of player careers as they really happened; choosing this option will result in annual player performance being fairly predictable. Allowing normal development means much more variable player careers as players improve and decline organically, and there may be some considerable departures, both positive and negative, from their actual real-life performance.