In BibtexFields there is a possibility to define an Extras string for each field type. I just replaced them with final variables in JabRef#374 and found some inconsistencies. The following values for Extras was identified and encoded, with comments:

• public static final String EXTRA_YES_NO = "yesNo";
  OK! Used by the IEEEtranBSTCTL entry in Better support for IEEEtranBSTCTL entries jabref#374
• public static final String EXTRA_URL = "url";
  Only used in EntryEditor. No field is assigned this in BibtexFields. Drop target for URLs.
• public static final String EXTRA_DATEPICKER = "datepicker";
  OK! Used to pop up a calendar. Set today's date on double-click
• public static final String EXTRA_JOURNAL_NAMES = "journalNames";
  OK! Used to show the abbreviation button.
• public static final String EXTRA_EXTERNAL = "external";
  OK! Open external viewer on double-click
• public static final String EXTRA_BROWSE = "browse";
  Only used in EntryEditor. No field is assigned this in BibtexFields. Gives a Browse button opening a file dialog.
• public static final String EXTRA_BROWSE_DOC = "browseDoc";
  OK! Given a browse button opening a file dialog with a predefined extension filter.
• public static final String EXTRA_BROWSE_DOC_ZIP = "browseDocZip";
  OK! Given a browse button opening a file dialog with a predefined extension filter, plus ext.bz2 and ext.gz
• public static final String EXTRA_SET_OWNER = "setOwner";
  OK! Gives a button that sets the owner to currect user.