How to define units non-physical/non-SI units for properties in AAS?

[Philipp-Liedl]

Is there a best practice approach to define units for properties which are not a physical or SI unit?

For example in an purchase order in which five pallets of a product are ordered, there is a property "OrderQuantity"=5 and the unit would be "pallet".

Should the unit in such cases be defined in the IEC61360, even if SI-based units are recommended to use, or rather in a separate property of type string called e.g. "OrderQuantityUnit"?