CDA (Clinical Document Architecture) is a version 3 document format and the CCD (Continuity of Care Document) flavor of it is being used more than any other type of HL7 V3 message in the USA. It is worth becoming familiar with the CCD if you want to put your hand into HL7 V3.
HL7 V2 is still the predominant way that data is exchanged in the USA, despite continuing attempts by the government to promote HL7 V3 data interchange. V2 is almost certain to remain predominant for the next several years and it may be difficult for V3 to replace it even after a few more years.
HL7 V3 is used extensively in Europe.
This answer was provided by a LinkedIn user.