This specific area is not my domain but I wondered if any of these would shed some light on the potential differences for you
https://www.securetechalliance.org/resources/pdf/CIV_WP_101611.pdf
Apologies if you have them or if they are wrong. We have been wrestling with other aspects of NHS documentation on their APIs which is slightly lacking and in a state of flux especially for their content services data quality.
Good luck, please post the answer here when you find it. We may be required to let the same happen for some of our services one day
Regards
David