Vacancies in positions in the classified service shall be filled by one of the following methods:
(1) Demotion
(2) Transfer
(3) Reinstatement
(4) Promotional appointment
(5) Competitive appointment
(6) Reemployment
(7) Temporary appointment
A vacancy shall be considered filled under any of the methods specified above, and employment thereunder effective, as of the date on which the employee enters upon the duties thereof.