Imagine that your company’s network owns the IPv4 range 184.108.40.206 – 220.127.116.11. The Reverse Lookup zone would be 60.50.40.in-addr.arpa, covering this entire network. Your primary nameserver is ns.acme.com. Your zone would look something like this:
60.50.40.in-addr.arpa ns.acme.com. support.acme.com. ( 20111202001 4h 1h 1w 1h ) @ IN NS ns.acme.com. 1 IN PTR www.acme.com.
But now you want to delegate the range 18.104.22.168 – 22.214.171.124 to the nameserver coyote.acme.com. Since it also resides in the same reverse zone, you cannot delegate this entire zone, as this would just move your entire network to the control of coyote.acme com, something you might not want to achieve with delegation.