when configure LAN via YaST I been ask for a hostname during the
configuration of a interface (LAN-Card).

And in the Hostname/DNS-Settings (one level above) I been asked for a
hostname and a domain name.

Why are there two locations to configure the hostname? And what is the
background for this double config? And last but not least: Should I
enter only a hostname or a FQDN in the interface settings?