The Publication Domain Name appears if you have the custom domain name feature and allows you to set the primary domain name for this publication. This domain name will be used in the "Launch" buttons within the site and also presented as the primary link for you to copy and paste from the account manager for this publication.

Please note that this does not force respondents to only use this domain when responding to the survey, it is simply a convenience so that the link you intend to use for this service is more readily available. The publication will default to using the primary domain name you have specified plan-wide if you do not configure it on a per-publication basis.