An initial serial number of DNS zone starts with 1 (one). It won't do for working installations. For example, one has a working DNS server with 2009071203 (YYYYMMDDNN as defaults everywhere) zone serial number. One installs CDS. Zone serial number sets to 1. One needs 2009071203 changes in the zone to get notifiers having learnt about changes. Till then, no new hosts, for example, will be known by world. Thus, one needs an option to set the zone serial number by hand.