Registry is a form of credentialing where a private organization gives their "stamp of approval" certifying you as a practitioner having a certain set of skills, usually by satisfying educational, clinical internship, and testing criteria.

Here is what our federal government has to say about it.

Certification and other qualifications. Although no State requires licensure in diagnostic medical sonography, organizations such as the American Registry for Diagnostic Medi-cal Sonography (ARDMS) certify the skills and knowledge of sonographers through credentialing, including registration. Because registration provides an independent, objective mea-sure of an individual’s professional standing, many employers prefer to hire registered sonographers. Sonographers registered by the ARDMS are Registered Diagnostic Medical Sonogra-
phers (RDMS). Registration with ARDMS requires passing a
general physical principles and instrumentation examination,
in addition to passing an exam in a specialty such as obstetric
and gynecologic sonography, abdominal sonography, or neuro-
sonography. Sonographers must complete a required number
of continuing education hours to maintain registration with the
ARDMS and to stay abreast of technological advancements re-
lated to the occupation

Registry Examination in Ultrasound (DMS) Source: United States Department of Labor, Bureau of Labor Statistics

Registry Examination in EchoCardiography   Source: United States Department of Labor, Bureau of Labor Statistics