Tom Adelstein writes about how to setup BIND on a Debian system.
Read it here
The early information contained in the primary zone file exists to accommodate your secondary or slave server. That's BIND for you. When you look at the SOA entry you will see the information for the secondary server in lines 2 through 6.
The primary zone file contains the bulk of the configuration information DNS needs. The zone file for the example.org domain is pri.example.org. (The pri prefix exists to help you recognize that it's primary.)