This code system http://hl7.org/fhir/v3/ActUncertainty defines the following codes:

CodeDisplayDefinition
Nstated with no assertion of uncertaintySpecifies that the act statement is made without explicit tagging of uncertainty. This is the normal statement, meaning that it is not free of errors and uncertainty may still exist.
Ustated with uncertaintySpecifies that the originator of the Act statement does not have full confidence in the applicability (i.e., in event mood: factual truth) of the statement.