Rule 729.4 · Subgames
All objects in the main game and all cards outside the main game are considered outside the subgame (except those specifically brought into the subgame). All players not currently in the subgame are considered outside the subgame.
Subrules
Some effects can bring cards into a game from outside of it. If a card is brought into a subgame from a main game, abilities in the main game that trigger on objects leaving a main-game zone will trigger, but they won’t be put onto the stack until the main game resumes.
A player’s main-game counters aren’t considered part of the subgame, although the player will still have them when the main game resumes. Similarly, any counters a player gets during a subgame will cease to exist when the subgame ends.