It's a standards of many standards. Each system (radiology, laboratory, scheduling, etc.) has their own system with it's own flavor of HL7. It then communicates

It's a standards of many standards. Each system (radiology, laboratory, scheduling, etc.) has their own system with it's own flavor of HL7. It then communicates with the larger hospital-wide system (Epic, Meditech, etc.) through interfaces that translate form the system specific to the flavor required by the hospital-wide system. Each interface implementation tends to make their own decisions about what certain pieces of information should look like, decisions that are specific to the healthcare organization and systems that they use.