The {CUSTOM_whatever <default>} tag is used to include the contents of custom fields in your messages.
For example, if you have been collecting your subscribers' fax numbers as described above, you could use the tag {CUSTOM_faxnumber unknown} in your messages. Like this:
Your fax number is {CUSTOM_faxnumber unknown}
This will either appear as
Your fax number is 123-456-7890 or Your fax number is unknown
depending on whether or not your subscriber's fax number is on file.