Unit of Measure
What a unit means, and what it converts to.
A conversion belongs to a class, not to a pair of codes. Metres and centimetres convert because both are length; metres and kilograms do not convert at all. Each unit says how many of the class’s base unit it is, so any two in a class convert through it.
Classes
None yet. Until there is one, every unit code on every part is free text and nothing can convert between two of them.