CDA is complementary to existing standards. It does not define new standards where existing industry standards are widely adopted or required by regulatory authorities.
HL7 standardizes electronic health record (EHR) data about patients. As CDA does not define standards for patient health data, there is no overlap with HL7.
CDISC standardizes patient data from clinical research for analysis and regulatory submissions. As CDA does not define standards in this area, there is no overlap with CDISC.