Yet, this plugin clarifies a lot:
During development, install it into your own application and you'll have a Services window showing the context of the global lookup, as well as the content of the central registry:
It enables you to, for example, see where your own implementations are registered in the default Lookup:
Above you can see I have two APIs, one for keyboards and the other for JFugue editors. That way, I'll end up providing a simple implementation of each when the product is released. Anyone else can then come along and replace my simple implementations with more complex ones.