This data type represents a boolean value which may be unknown. In other words, this data type can represent Yes, No, and Unknown; unlike a traditional boolean which can only represent True and False. For this logical model, this class has been modeled in the HL7 V3 style, which has the advantage of being able to account for different kinds of "unknowns", but has the disadvantage of having two fields, one of which must always be populated while the other must always be empty. Implementers may choose to implement this class using a single field instead.