What does it mean to link prefabs?

I have read over some of the best practices regarding prefabs but it is all very wooly in the area around managing multiple prefabs via composition. So for example lets say you have a Player prefab, which contains a child model and a child behaviour tree. Now if you want the behaviour tree to be its own prefab then the moment you attach it to the player prefab it loses its connection. So it was mentioned a few times to link prefabs, but what does this actually mean from an implementation point of view? http://devmag.org.za/2012/07/12/50-tips-for-working-with-unity-best-practices/ That is the article that specifically says it.