This code system http://hl7.org/fhir/filter-operator defines the following codes:

CodeDisplayDefinition
=EqualsThe specified property of the code equals the provided value.
is-aIs A (by subsumption)Includes all concept ids that have a transitive is-a relationship with the concept Id provided as the value, including the provided concept itself (i.e. include child codes)
descendent-ofDescendent Of (by subsumption)Includes all concept ids that have a transitive is-a relationship with the concept Id provided as the value, excluding the provided concept itself (i.e. include child codes)
is-not-aNot (Is A) (by subsumption)The specified property of the code does not have an is-a relationship with the provided value.
regexRegular ExpressionThe specified property of the code matches the regex specified in the provided value.
inIn SetThe specified property of the code is in the set of codes or concepts specified in the provided value (comma separated list).
not-inNot in SetThe specified property of the code is not in the set of codes or concepts specified in the provided value (comma separated list).
generalizesGeneralizes (by Subsumption)Includes all concept ids that have a transitive is-a relationship from the concept Id provided as the value, including the provided concept itself (e.g. include parent codes)
existsExistsThe specified property of the code has at least one value (if the specified value is true; if the specified value is false, then matches when the specified property of the code has no values)