Depends if both are Quantities, for example the state of Number:Length Items, ar a constant defined like 3 | mm.
If both are quantities of the same type, just subtract, the framework sorts it all out including units conversions.
You can force the (quantity type) result into units of your choice.
If they’re mixed quantity / plain number types, you have to take action to force similar typing.
Note that myQuantityItem.state as Number
may not give you what you expect.
