What gets saved

Which objects

Object	Needs a Host Migration Id?
Placed actors	Yes
Actors spawned at runtime	Yes. They are spawned again on the new host.
Player pawns	Yes
PlayerStates	No. Matched to the returning player by unique net id.
GameState, GameMode	No
Components	No. Saved with their owner.

A placed actor that was destroyed before the migration stays destroyed on the new host.

Which data

• SaveGame variables. Tick SaveGame on Blueprint variables (Advanced section), or use UPROPERTY(SaveGame) in C++. Structs, arrays, maps and object references are supported.
• Custom data (C++). Implement IHostMigratable for anything SaveGame cannot express.
• The actor's transform is always saved.

Components

Every component with SaveGame variables (or IHostMigratable) is saved with its owner: default components, components added in the Blueprint editor and components added at runtime. This works on actors, PlayerStates, the GameState and the GameMode alike, so an inventory component on the PlayerState just works.

Components are matched by name. A component that was added at runtime and is missing on the new host is created again with the same name and class.

References

Object references are stored so they still point at the right thing after the migration:

Reference to	Stored as	After the migration
An actor with a Host Migration Id, or one of its components	Migration id	Points at the same actor, even if it was spawned again.
A PlayerState	Unique net id	Resolves if that player is connected when the object is loaded. Otherwise null.
Assets, classes, placed actors without an id	Path	As before.

Raw pointers, TObjectPtr, TWeakObjectPtr and references written through IHostMigratable all work.

What is not saved

• Variables without the SaveGame flag.
• Actors without a Host Migration Id.
• Timers, animation state, physics velocity, in-flight RPCs. Save what you need in On Host Migration Save and restart it in On Host Migration Restored.
• Progress made after the last snapshot (at most one Snapshot Interval). A graceful leave sends a final snapshot.