Sound Effect Set
Similar to how the HitObjectSet works, SoundEffectSet is a ScriptableObject asset that groups together all sound effects used.

Each one is played for a particular situation. Check the tooltip descriptions for more info.

Warning
MouseDance does not implement the recognition of custom hitsounds coming from the .osu file yet. Check the Roadmap and Release Notes for updates.
Custom Sound Effect
Custom Sound Effect is used in custom rules. When customizing Collect Rules, you can optionally make the game use a custom sound effect.