| Code | Display | Definition |
| literal | Literal References | The server supports and populates Literal references where they are known (this code does not guarantee that all references are literal; see 'enforced') |
| logical | Logical References | The server allows logical references |
| resolves | Resolves References | The server will attempt to resolve logical references to literal references (if resolution fails, the server may still accept resources; see logical) |
| enforced | Reference Integrity Enforced | The server enforces that references have integrity - e.g. it ensures that references can always be resolved. This is typically the case for clinical record systems, but often not the case for middleware/proxy systems |
| local | Local References Only | The server does not support references that point to other servers |