Environments can be added to the game to change or add rules to the overall game play.
Each Environment shows a list of potential things it can do at game setup. Each item in the list has a percentage that shows the chance that this particular item will be chosen when the Environment is included in the game.
Ladder and Casual matchmaking automatically may include Environments that the players' own. For Practice and Friendly games, you must either have created Sets in the Set Builder that include the Environments you want, or choose "Random (with Environments)" from the Set Picker.
In game, you can see what Environments are active by pressing in the pull-out tray.
The Environment's chosen item is shown with a check mark in front of it, and the non-chosen items are dithered.