XADM:

When you export the settings of a Mailbox, you may see the Hide from AB field. This field is used to determine whether or not a Mailbox is visible in the global address list. By default, when Mailboxes are first created on an Exchange server, they are not hidden from the address book. The Hide from AB field is empty when you export it.

You might expect that, by default, this field would be set to 0 for FALSE. However, if you have never modified the setting on the Advanced tab of the Mailbox properties, this field setting is empty.

There are three possible settings for this field: You will only see a value of 0 if the Mailbox has been hidden from the Address Book in the past, and then unhidden. This has no effect on the performance of Exchange, but this may be a factor to consider when you create scripts to run against exported .csv files that contain user settings.