Defining additional names of subsets of characters in fontconfig.properties in jre

Question

Java documentation on the status of the font configuration file:

CharacterSubsetName is the name for a subset of the Unicode character set that can be displayed by some composite fonts. For Windows, the following names are predefined: alphabetical, Arabic, Chinese-ms936, .....................

(the list goes on)

"The font configuration file may specify additional names to identify additional subsets of characters."

What is the format and where in the fontconfig file should additional names be defined for additional subsets of characters?