Looking at airfares.com.sg site for some SQ flights, I see the following rules:
http://www.airfares.com.sg/airfares/..._china_sha.htm

Permitted Combinations :
--End - on combination is not permitted
--Round trip / circle trip combinations permitted
--Turnaround open jaw journey for the same fare type is permitted between 2 cities as stipulated on the fare provided the
higher return fares must be charged
etc etc....


Any, my question is, what does end-on combination not permitted mean?