XML tree structure

The input and output format lists use trees to show message structure. Although details may differ from format to format, all trees fall into two categories:

  • Simple message formats.

    These organize data into trees that maintain fixed parent/child relationships. That is, if a child is shown in the tree, it displays in the message. These formats tend to have only a few permissible generational levels of parents begetting children, who beget children, and so on.

  • Hierarchical message formats.

    These organize data in a more complex manner. Generational depth is not necessarily restricted. Portions of a message displaying in a tree may be optional, and portions of a tree may repeat in the actual message. This type of tree is called HMD, or Hierarchical Message Data. One such HMD format is HL7. Optional portions of the tree are indicated by bracket pairs ( [ ] ). Potentially repeatable portions are indicated by brace pairs ( { } ).

    HMD respects definitions based on the priority of local site, master (primary) site, and root.