MusicClassifications are used to create hierarchical relationships between MusicLayouts that allow overriding music based on game state. Here is an example: In Metroid games, areas will typically have music associated with them. However, when the self-destruct sequence starts, only the self-destruct music plays no matter what area you enter. MusicClassifications would allow you to classify the self-destruct music as higher priority and would therefore override the default area music. If the self-destruct sequence ends, you can also revert to the area music because the triggers were still cached.
Higher values are higher priority.