You can define a unit as a multiple of an existing unit or combination of existing units.

A simple example is that you might want to define cL (short for centilitre) as 1/100th of a litre. Often useful is to define a new name for a combination of units, for example you might want acc (short for acceleration) as a short name for m/s^2.You should only use this to define relationships which are fixed (e.g. 1cm = 0.01m). For example, you could define a Euro to be 1.1 US dollars, but the exchange rate changes so it is not a good idea to fix it in the unit definition.