Thanks for noting this, Ryan
Yes it does correspond to that draft lpo: ontology, a Linked *Pasts* 
Ontology, thought at the time to be ultimately broader than for Linked 
Places format
The rdfs:comment for that is incorrect. As stated, any string is okay, 
making 'preferred value' meaningless.
Not sure how one would restrict allowed values for an xsd:string, or 
whether that is even possible. Maybe needs owl:withRestrictions?
But in the LPF spec we can say that only values ‘certain’, 
'less-certain', and ‘uncertain’ are allowed for software supporting LPF. 
I should have included that.
The lpo: ontology is an early draft and does need revisiting!
kg