I've just put up a draft secondary specification on how we could
represent parameter uncertainty within the context of my CellML 1.2 draft.
The secondary specification can be viewed here:
The DocBook it is generated from can be checked out here:
It is written against this draft CellML 1.2 specification:
It builds upon, and refers to, the draft secondary specification for
models of differential-algebraic equations with optional events,
cellml-discussion mailing list