What’s the purpose of the Curves layer at the top of the Subject [Layers] folder?

This layer is required to ensure that the layers contained within the folder display correctly when viewed in Photoshop’s native color space. You can safely ignore it - for the best results just leave it in place and don’t change the curve. To do your own colour work, just add your own adjustment layers as you normally would. To color correct the entire Subject like you would a conventional image, just add your adjustments above this Curves layer.