[ubl-lcsc] Document instance UUID

Should we add a placeholder for UUID (Universally Unique
IDentifier) definition to all 7 document types?

These UUIDs are meta data perculiar to the document instance
(not the document type), and are generated by UBL-processors
at the point when the document instance is created.

Independent ERP/MRP/DP systems may have their own system of
issuing serial IDs, but there is no guarantee that the IDs
will be unique across other systems, companies, countries.

The UUIDs offer characteristics that allow such purposes
as document instance "equality test", indexing and tracking, 
to be carried out in a more well-defined manner.

In addition, using UUIDs, we might also then distinguish
between document duplicates (different UUID but everything
else may be the same values) and repeated sends (same UUID
used in the files) of the same document.  

For instance, in real life, duplicates might need to be sent
by administrators due to various transport reasons (or others)
such as file received corrupted/incomplete, interim network 
disruption/failure, etc.  Repeated sends might be initiated by 
business managers and have business implications as they could 
mean genuine reminders or a contractual obligation to send 
reminders prior to taking some business actions.


Regards,
Chin Chee-Kai
SoftML