Effects - Ballpark effects are ranked on a scale from - 4 to + 4, based on their impact on hits (1B), doubles (2B), triples (3B), homeruns to left field (HR LF) and homeruns to right field (HR RF). These values are based how the ballpark has historically performed in each of these respects as compared to its contemporary ballparks. A negative number indicates that the ballpark yields less than the average for the type of hit, while a positive indicates that the ballpark yields more than the average for the type of hit. The more negative or positive number, the more extreme the effect for that hit in that direction. These numbers are just indicators of the relative effect in the simulation engine. They are important for users and founded entirely on historical fact, but are meant for display purposes and not explicitly used in the simulation engine.

The above is from the help section. I never noticed it before, but a new HBD coach asked me about it.  I assume its a typo, and its talking about park dimensions, not park effects, but does anyone know for sure?

I'll try to reword my previous response.

I take "not explicitly used in the simulation engine" to mean that in the engine code (it is a computer program), they are not using "-4" or "+4" as a modifier in the calculation of the outcome of a play.  Whatever they did to simulate park effects is probably a little more complex than that.  They just represent the relative effects as a -4 to +4 scale so that one can easily differentiate between "pitcher's park" or "hitter's park".

These numbers are to give you an idea of what to expect with the ballpark. They are not the exact numbers that go in but are relative to what is considered.