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From an OpenMI processing/adaptor point of view, the dimensionless unit is of little use (as I see it, unless I am missing something). It is not possible to do any automatic conversions from kg/kg to m^3/m^3 anyway. It can have some use in validation, in order to check that you are linking items that have the same non-dimensionality. Otherwise the dimensionless information could also just be provided as information in the description of the quantity, and then leaving the standard as it is now.
A flag indicating dimensionlessness could be added to the dictionary extensions 
2.3. Mixing time-dependent and time-independent components
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