Ok so no response from HB for a legit question on the game. Shame as the description of this setting doesn't really clearly explain what it means to adjust it.
Anyways looking through stats of a season of playing with this set as 95% compared to 50% it appears it is the % of the field that are the better players. Higher the %, the more of the better AI players make up the field making it harder to make the cut AND making the field a bit tighter it seems.
At 95% when checking the stats for FedEx points, only around 180 players play over 10 events in a season so meaning each event has more of the top players, similar fields and likely tighter fields whereas at 50% you're looking at all but a few playing over 10 events (around 250 players) and so leading to more different fields from event to event which leads to differing scoring in each event so more potential to win an event and more stretched fields depending on who of the top players are chosen to play.
Need to find my balance of number top players to make the field tighter and a bit more challenging whilst maintaining realistic scoring yet have enough diversity so I'm not just playing against 180 players to make the top 125 for the playoffs.
While playing with AI difficulty at 82 (around -5 to -6 for max round scoring), I am trying 85% participation to see what this brings. Hoping for a) around 200 players playing over 10 events in a season making it a bigger pool of players that could make the top 125 for the playoffs yet b) have enough top players in each event to make the field tighter and make it challenging to make the cut let alone the top 20.