The only lines that do not conform to the standard RR format in a data file are control-entry lines. There are two kinds of control entries: $INCLUDE() and $ORIGIN().
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.
The $ORIGIN command is a way of changing the origin in a data file. The line starts in column 1, and is followed by a domain name. It resets the current origin for relative domain names (for example, not fully qualified names) to the stated name. This is useful for putting more than one domain in a data file.
You cannot use $ORIGIN() for putting more than one zone in a data file.
Use of $ORIGIN commands in a data file is optional. If there is no $ORIGIN() statement the default origin for DNS data files is the domain named in the second field of the primary or secondary line of the named.conf file.