A vacancy in a service post shall be filled

(a) by promotion, that is by appointing a civil servant who will move from one grade to a higher grade with an immediate increase in salary,

(b) by transfer within the Service, that is by appointing a civil servant who will move from one grade to another grade without an alteration in salary,

(c) on reduction in status, that is by appointing a civil servant who will move from one grade to a lower grade with an immediate reduction in salary,

(d) by recruitment, that is by appointing a person

(i) who is not a civil servant to the service, or

                     (ii) who would cease to be a civil servant if the appointment were not made.