The name of the unit of measurement.
Every unit must have name, and it cannot be the same name as other units (including built-in units like m for metre). Unit names must start with a letter or currency symbol (e.g. $) and can only contain letters, underscores and currency symbols. They cannot contain numbers or spaces.
This creates a new unit (rather than just a new name for an existing unit).
Units can be defined as a multiple of another unit (e.g. a millimetre is 1/1000 of a metre) or totally independent of existing units.
Your description of the new unit, for your own reference.
Describe what the unit is, so you can remember later on what it is for.
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.
Makes a new name for an existing unit.
This makes a new name for an existing unit, for example you might want $ as a short name for USD. The key difference between using an alias and creating a new unit is that if you want to swap between two separate units, you must use a conversion function, whereas with an alias you do not. So if you have two units that are just different names for the same unit (for example, a shorter abbreviation for inch), use an alias. If you have two different units that happen to be equivalent (for example, you might want a unit bagOfSugar which equals 1 kilogram, but they are not measuring the same thing), define a full unit instead.
The target for the alias.
For example, if you want to define f as a shorter name for foot, put "f" in the name field above, and put "foot" as the target in this field.