First of all ‘Self-Service Site Creation’ service is available only for WSS 2.0 top level site in ‘/’ not for Portal Site. So this right is only visible in WSS 2.0 top level site in ‘/’ but not in Portal site even service is enabled.
We cannot use ‘Self-Service Site Creation’ for the Portal site like ‘http://server:Port/ default.aspx’ even it is top level site in path ‘/’. It must be WSS site. And more important is for PORTAL sites like ‘http://server:Port/default.aspx’, there is no meaning of Enabling/Disabling ‘Self-Service Site Creation’. There will be no facility and NO RIGHTS about ‘Self-Service Site Creation’ available in RIGHTS list UI. Programmatically, this can be verified by reading PermissionMask of the group we created and ANDing with individual RIGHTS.
‘Self-Service Site Creation’ is the Site RIGHTS and will be available in UI only for the Top-Level WSS Site in ‘/’ (E.g. ‘http://server:Port/default.aspx’) where it was enabled. When it is enabled, there an announcement will be added to the Announcements list on the home page of the top-level Web site. The announcement will provide a link to the site creation page (scsignup.aspx in the _layouts directory).
Among default five groups: Reader, Contributor, Web Designer, Administrator and Guest (Invisible in UI), by default it is enabled for all groups expect Guest. However, we can enable/disable later for Reader, Contributor and Web Designer (AND any group we added) if site support this feature. However for others Portals also, Permission Mask is same for default groups as WSS 2.0.