| Code | Display | Definition |
| grouped-by | Grouped By | No particular relationship between the concepts can be assumed, except what can be determined by inspection of the definitions of the elements (possible reasons to use this: importing from a source where this is not defined, or where various parts of the hierarchy have different meanings) |
| is-a | Is-A | A hierarchy where the child concepts have an IS-A relationship with the parents - that is, all the properties of the parent are also true for its child concepts |
| part-of | Part Of | Child elements list the individual parts of a composite whole (e.g. body site) |
| classified-with | Classified With | Child concepts in the hierarchy may have only one parent, and there is a presumption that the code system is a "closed world" meaning all things must be in the hierarchy. This results in concepts such as "not otherwise classified." |
