ubl and extension

Hi. I've heard talks and read papers about how UBL documents are intended
to be extended in verticals with information specific to that vertical.
This is the notion of context, I believe.

Perhaps it is still undecided, but I am wondering at what level of the
document is it intended that the extension take place?

As a simple example, if I wish to extend a UBL Order so that the
information specific to my industry is expressed, should I extend the
OrderType? Or do I extend something further down in the document, like the
LineItemType? Or should I just extend whatever I need to extend to get the
job done? Are the documents being designed with the intent that certain
types/elements are natural extension points, and other types/elements
should probably remain consistent across verticals?

thanks,

Patrick