The system uses the units of measure system to convert currencies as well as general units. Currency exchange rates tend to change quite often. To make it easier to handle this you can define conversion expressions that reference variables. A variable is a number that you give a name to. You can reference these named numbers by their name from within any conversion expression.

For example, say you had a pair of conversion expressions to convert between Pounds Sterling and US Dollars. You could define a variable called DollarRate, and the conversion expressions as:

Pounds = Dollars / DollarRate

Dollars = Pounds * DollarRate

Then if the exchange rate changes, all you have to do is change the value of DollarRate and both conversions change automatically.