Starting this year, Double-A starting pitchers – those on the 40-man roster with a majority of their time spent at Double-A – cannot participate in winter ball if they threw 140 or more innings. The previous threshold was 155. Similarly, Double-A relievers cannot have appeared in more than 45 games, while in years past it was 55.
Pitchers also are ineligible if their workload in games or innings grew 25 percent over the prior season. The caveats: They must've reached 60 percent of the games-or-innings threshold the previous season, and players switching from the rotation to the bullpen or vice versa don't count.
Instead of a 502-plate appearance threshold for major league position players, those with up to 552 now can play winter ball. Moreover, the past procedure of declaring a "physical incapacity" – teams simply had to provide documentation – now has strict rules. Unless a player finishes the season on the disabled list or spent 60 days on the DL (including 15 over the final 60 days of the season), a team cannot declare him physically incapable. The exception is pitchers who have undergone major surgery in the previous 18 months. — Jeff Passan, MLB reaches deal to allow players to participate in winter leagues
I generally don't like all these restrictions MLB and teams put in place, but these at least seem reasonable. Except for the relievers. I don't understand that at all, but I also don't understand why teams refuse to use relievers more effectively than they do.