use of 'ID' in documents

We notice that some documents contain an 'ID' element.  The 'ID' 
elements seem to be mandatory.  The documents that don't have 'ID' seem 
to have 'BuyersID' and 'SellersID', but those are optional.

Is this an intended modeling aspect that assumes in the documents that 
don't have 'ID' the users would implement either a BuyersID or 
SellersID, even though they are both optional?  Do we want to duplicate 
this model for new documents?  When do you determine you want a 
mandatory ID vs. two optional Buyers|Sellers IDs?

Thanks,
Anne