https://www.mlb.com/interactive/mlb-abs-system-explainer
“Simplifying to two dimensions
Think it’s hard to call a ball or strike just based on the height and width of the zone? Now throw in the third dimension – of where over the plate the zone should be called.
While the rule book has long officially defined the zone as being a three-dimensional pentagon shape hovering over the plate, ABS testing in the Minor Leagues led to unsatisfying results when using the 3D zone. Therefore, the ABS zone you’ll see in the Majors is a line set at exactly the midpoint of the plate, 8.5 inches deep. This change eliminated some odd outcomes in testing and produced results more in line with the expectations of players and fans.”