The unit of measure system is used for all quantities. The quantities can be engineering quantities; metres, bars, sheets, etc., or currencies; sterling, dollars, euros, etc. The unit of measure system always knows what the units of a quantity are, as well as the quantity itself. It also knows how to convert to and from the most common units of measure. This means you don’t have to worry about conversions, the system will do it for you automatically. This is particularly significant when dealing with currencies. If you are dealing with foreign customers and suppliers, it can convert to and from their currencies in the appropriate places. All you have to do is update the conversion rates from time to time.

Most of the time, the units of measure system just works away in the background and you will not be aware of it. The only time you are likely to be made aware of it is if you define a new unit of measure and don’t tell the system how to convert other units to and from it. If it finds it needs to make a conversion and doesn’t know how, you will receive a notification message and/or the result of the conversion will be undefined.

It’s critically important to realise that units of measure have a meaning; they are not just numbers. This is significant if you try to specify quantities in meaningless or nonsense ways. If you’re lucky the system will notice straight away and give you an error message that will point you at the problem. More likely, the error will not be noticed until the system comes to use it, while planning an order for example. This may be some time later and from the immediate ‘effects’ of the error it may not be obvious where the root of the problem is. Be careful and responsible when defining quantities.

When the system performs arithmetic involving different units of measure it is very fussy about being able to convert between them. If it cannot the result is always a special number referred to as undefined. The result of any arithmetic involving undefined is also always undefined. So, for example, if you have a complex product structure that uses a screw that costs 10p per 1000 and you haven't assigned a cost to that screw, then the total cost of your multi-million pound project will be calculated as undefined!