It seems as though the "mandatory training" decree that came down from WWE towards Performance Center talent. However, the reaction to this wasn't positive and, according to this report from Fightful, it looks like they've lightened up on it.

We've heard some more about what was originally set to be mandatory WWE Performance Center training that took place before WWE returned to the road. While it was first announced that the training would be mandatory, that changed in the days following, as talent relations made it clear to talent that if they had a personal reason for not attending, that would be taken into consideration. As it turns out, several wrestlers cited personal reasons and numerous talent were not in attendance. One talent specifically put over how almost all of the WWE's female talent made the trip in.

This was the rule before Bayley's injury, and it looks like WWE are going back to it.