Because the Major League Soccer season was already halfway complete when when the International Football Association Board instituted its latest series of rules changes last year, MLS games continued to be governed by referees without those changes in 2019.
That changes in 2020, with the new season seeing MLS adopt those new changes. Those include some adjustments in the protocol for Video Review, outlined in the above video by PRO's Manager of Video Review Greg Barkey.
If you've watched international matches like the 2019 FIFA Women's World Cup or games in this current European season, you're probably already familiar with many of these changes. But it's always helpful to get a refresher before opening weekend: