Branch objects in Multiple Endings scene have one specific setting "Unlocks after linked level completed", which can be used to create a level map. In level map, player can see some levels and only the first one is unlocked. After completing the first level, the second unlocks, etc.
To achieve this, you need to create a new Multiple Endings scene and add some branches.
Then you need to decide what scene sequence you want to build. In our example, only Level 1 is unlocked at the beginning. Level 2 unlocks when the player completes Level 1. Level 3 and Level 4 unlocks after Level 2 is completed, and Level 5 unlocks after Level 3 is completed.
Here is how you can do this: to make Level 3 locked, open its settings, find "Unlocks after linked level completed" and select "Level 2".
The blue lock will appear on Level 3 branch object indicating that this branch is locked with other scene.
You can also have forks in your level map. In our example, both Level 3 and Level 4 are unlocked when player completes Level 2, this way the player can choose what levels he wants to play next.