diff between "option domainname" and "ddns-domainname" ?

[kalyanasundaram S]

what is difference between These two
  Option domain-name 
  ddns-domainname.

I was doing for ddns updated via dhcp. I was not sure what the client will send for the host name.
so i made a host declaration
host test {
  hardware ethernet oo:11:23:...;
  ddns-domainname "example.com";
  ddns-hostname "abcd";
}

what i did was 
i mention option domain-name "example.com"; and restarted the dhcp server and looked for abcd.example.com
it is not available and ran dhcpcd on the client side after that abcd.example.com was available on dns.

Now instead of " option domain-name " i mentioned " ddns-domainname " again restarted the dhcp server.
for the sake i changed host name abcd to xyz and address range too.
Did not do anything on  client machine. Just checked for xyz.example.com . It is available.
How come this is being updated before the client gets that name...

or this is because of host declaration. If then why this did not happened when it was "option doman-name"
How dhcp server will consider these things...

How do i make the client to send a host name properly.

can anybody help on this?. plz.
thanks in advance...
  -"kalyan"