The word khalifa is an Arabic word meaning successor. That is why Hazrat Abu Bakr (r) was called khalifa because he succeeded the Holy Prophet Muhammad (pbuh). When he passed on and Hazrat Umar (r) became the head of Muslim state, a new title had to be found for him and the term ‘Amir ul Maumineen” was coined.
Earlier Kalifa’s of the Qadiani jamaat called themselves “khalifa tul Massih” or successors of the Promised Messiah. More recently however they have started calling themselves Khalifa tul Islam – meaning head of the Muslim community. This is laughable because only a tiny percentage of Muslims are members of their jamaat. But it is also sinister in that it implies that only those who are members of their jamaat are Muslims. Earlier Qadianis used to openly say that those not in their jamaat are heretics. They have now abandoned that but are now using more subtle methods to call others heretics.