An include file is any file named in an $INCLUDE() statement in a DNS data file. $INCLUDE files can be used to separate different types of data into multiple files for your convenience.
For example, suppose a data file contained following line:
This line causes the /etc/named/data/mailboxes file to be loaded at that point. In this instance, /etc/named/data/mailboxes is an $INCLUDE file. Use of $INCLUDE files is optional. You can use as many as you wish, or none at all.