Item relationships - Planning for external systems

Summary:

There is a file Supply Chain Planning Item Substitute to load an item substitute is an alternative item or component that can be used in place of the original item.

There is also a Item strcuture file where we can define substitutes for components. 

While it is clear how Item structure substitute is used by constraint Plan, could anyone explain how the first file Supply Chain Planning Item Substitute is being used by planning. Also, Does it mean end item substitution?

Also, In some of the older posts, I can see end item is being considered for development. Any guidance on the timeline.