This variable specifies one or several class names to be used for custom variables, in the form of a comma-separated list. A custom variable is a variable not normally known to PostgreSQL proper but used by some add-on module. Such variables must have names consisting of a class name, a dot, and a variable name.
custom_variable_classes specifies all the class names in use in a particular installation. This parameter can only be set in the postgresql.conf file or on the server command line.