n - Name used to reference the field, especially when writing the .dat file.
pn - Label that will be visualized by the user. It can be translated.
p - Priority of the unit, it is regarded as more important than others units. The possible values for 'p' are 1, 2 and 3. For more information about this attribute, see the sectionDescription of the units
factor - The conversion factor used to multiply a quantity when converting from one system of units to another. For more information about this attribute, see the sectionDescription of the units
help - To make easier to the user to identify the unit.
units_system - There are two systems used to define units, the possible values are:
SI - International system
imperial - Imperial system
addend - Conversion number, relation between two units. Example: In order to go from Celsius to Kelvin, 273.15 is added to the temperature in Celsius. Therefore, addend = 273.15.
printable_name - Name used to print the unit. For instance, 'oC' could be the print name of ºC.