Preferences are miscellaneous session information. They include, by default, the following fields:
email fax phone_night fax_order email_copy
The field p_nickname acts as a key to select the preference set. To change the values that are included with the preferences parameter:
[userdb function=set_preferences preferences="email_copy email fax_order fax"]
or in catalog.cfg:
UserDB default preferences "mail_list email fax_order music_genre"