Expected slugging percentage (xSLG) is formulated using exit velocity and launch angle, two metrics measured by Statcast.
In the same way that each batted ball is assigned a Hit Probability, every batted ball has been given a single, double, triple and home run probability based on the results of comparable batted balls -- in terms of exit velocity and launch angle -- since Statcast was implemented Major League wide in 2015.
All hit types are valued in the same fashion for expected slugging percentage as they are in the formula for standard slugging percentage, with doubles being worth twice as much, triples being worth three times as much and homers being worth four times as much as singles. The single, double, triple and home run probabilities for an individual batted ball are plugged into the formula for slugging percentage -- (1B + 2Bx2 + 3Bx3 + HRx4)/AB) -- to get a player's expected slugging percentage on said batted ball.
Knowing the expected outcomes of each individual batted ball from a particular player over the course of a season allows for the formation of said player's seasonal expected slugging percentage based on the quality of contact, instead of the actual outcomes. Likewise, this exercise can be done for pitchers to get their expected slugging percentage against.
Why it's useful
Expected slugging percentage is more indicative of a player's skill than regular slugging percentage, as xSLG removes defense from the equation. Hitters, and likewise pitchers, are able to influence exit velocity and launch angle but have no control over what happens to a batted ball once it is put into play.
Watch: Manny Machado records a batted ball with a lofty expected slugging percentage but is robbed by Kevin Pillar.