The manual states:

"Only a very few PURE BASEBALL ratings are actually used in the game simulation. The ratings are included in the game to help managers quickly and easily identify a player's strengths and weaknesses. The actual statistics are what drives the game in most cases. Example: A pitcher may have a 7.3 pitching rating, but his actual statistics versus the type of batter (RH or LH hitter) he is facing, the batter's statistics, the park effects, and other pertinent factors determine the outcomes"

I read this to mean the park effects of the park where the game is played are used along with the pitchers stats vs the type of batter plus the batters statistics to determine the outcome.

I do not read it that both the pitcher and batter are adjusted for the park effects for the parks where they played (adjusted for multiple parks if they played for multiple teams?) and then that there is an additional adjustment made based on the park where the current PB game is being played.