This case-sensitive code system http://hl7.org/fhir/match-grade defines the following codes:

Code	Display	Definition
certain	Certain Match	This record meets the matching criteria to be automatically considered as a full match.
probable	Probable Match	This record is a close match, but not a certain match. Additional review (e.g. by a human) may be required before using this as a match.
possible	Possible Match	This record may be a matching one. Additional review (e.g. by a human) SHOULD be performed before using this as a match.
certainly-not	Certainly Not a Match	This record is known not to be a match. Note that usually non-matching records are not returned, but in some cases records previously or likely considered as a match may specifically be negated by the matching engine.