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PQS SI system units classes
The SI system identifies 2 classes of unit, coherent units and non-coherent units 1. The SI also acknowledges other units outside the si which are associated with si units by means of conversion factors, which are also non coherent units .
Coherent units consist entirely of some product of base_units. There is no use of conversion factors (including integer powers of 10).
Incoherent units are related to coherent units by means of a conversion factor which can convert their numeric value to some value expressed in coherent units.
The SI distinction into coherent and incoherent units is slightly confusing since it explicitly defines some incoherent units with conversion factors consisting of powers of 10. e.g kilometers megawatts, while other units ( usually legacy units, often with conversion factors that are not a simple power of 10) are excluded, therefore PQS classifies units for use with the SI system into 3 classes:
PQS defines another class of units which includes non-coherent units that are not normative units. This class includes all the units from other measurements systems , such as the 'foot' length unit. It also incidentally may include units that happen to look normative in terms of their conversion factor but represent units in another unit system. The SI system considers these units second class citizens and does not recommend their use in calculations.