Philosophy behind the CVA contracts

I have a question I should have asked myself earlier... How does one 
make the links between his own set of codes and the ones in the CVA 
agreement (which might be different)?

Example: A participant uses internally 'SP' as the country code for 
Spain whereas he received documents with the code 'ES' for Spain (as per 
a CVA agreement). How can the system know both codes relate to the same 
'Spain'?

Eric Desgranges.