From an AP story by Ronald Blum headlined “What is a strike in baseball? Robots, the rule book, and umpires view it differently”:

The education of robot umpires has been complicated by an open secret in baseball for the past 150 years: The strike zone called on the field doesn’t match the one mapped out in the rule book.

Before the Automated Ball-Strike System is ready for the major leagues, there has to be agreement on what a strike is.