An include line begins with $INCLUDE in column 1, and is followed by a file name (known as the $INCLUDE file). This feature is particularly useful for separating different types of data into multiple files as in this example:
The line is interpreted as a request to load the /etc/named/data/mailboxes file at that point. The $INCLUDE command does not cause data to be loaded into a different zone or tree. This is simply a way to allow data for a given zone to be organized in separate files. For example, mailbox data might be kept separately from host data using this mechanism.
Use of $INCLUDE statements and files is optional. You can use as many as you wish, or none at all.