@bna What does ‘uncertain’ mean? In what context - data presence/absence or clinical certainty. Serious question. We probably all use it differently unless it’s attached to a value set.
In the archetype modelling we only use Boolean data types when we are sure there is only a Yes/No or True/false clinical answer. Then the null flavours are related to whether data is available or not.
In clinical modelling reality we hardly ever use Booleans, because there are hardly any clinical situations where there is a clear black and white answer. Too often there are subtle shades of grey, which I suspect might be one of the reasons driving your request for ‘uncertain’. In those ‘grey’ situations in archetypes, we tend to use the pattern of CODED_TEXT with a codable value set ‘Present/Absent/Indeterminate’. In this context ‘indeterminate’ means ‘we looked but we couldn’t tell’ - and that could be because of inexperience or simply that it could not be discerned even by the most experienced clinician, but it is still a very important and valid clinical finding that needs to be recorded in its own right, not as a RM ‘flavour’.
Just throwing this into the mix and stirring the soup 
Cheers