Blackberry 8800 User Manual page 166

Custom address book field data is not appearing on my computer
You can only synchronize the data in a custom address book field with a text field in the email application on your computer. For example,
you cannot add a contact's birthday to a custom address book field on your BlackBerry® device and synchronize it with the email application
on your computer, because the birthday field is a date field in the email application on your computer.
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