This code system http://hl7.org/fhir/graph-compartment-rule defines the following codes:

CodeDisplayDefinition
identicalIdenticalThe compartment must be identical (the same literal reference)
matchingMatchingThe compartment must be the same - the record must be about the same patient, but the reference may be different
differentDifferentThe compartment must be different
customCustomThe compartment rule is defined in the accompanying FHIRPath expression