Do we have any guidance around what IFC considered as co-planar? Let me give a concrete example. Imagine that you have a faceted BRep (of any of the acceptable IFC2x3/IFC4 variants). One of the facets is defined by 4 points - (0,0,0), (0,1,0), (1,0,0) and (1,1,eps), where eps is a small number. Clearly if eps = 0, these 4 points are co-planar, and if eps is anything else, they mathematically are not. For what values of eps would we say that the incoming data is incorrect (e.g., I would think eps=1 is clearly invalid), and for what values would we say that the receiving software should accept the information as representing a plane?
On a bigger note, I think these sorts of issues will become more prevelant when IFC4 advanced BReps are used more often, but hopefully we can start somewhere.