70 ILCS 2605/4.10
The Director shall note of record the duties (whether imposed by law, official regulation or practice) of each classification in the classified service, and shall thereupon by rule fix lines for promotion from lower classifications to higher classifications in all cases where, in his or her judgment, the experience gained in the lower classification may tend to qualify an employee to perform the duties of a higher classification. In case of vacancy in higher classifications, which cannot be filled by reinstatement, the Director shall hold promotional examinations to fill such vacancy. Incumbents of classifications in lines of promotion established by the Director shall be solely eligible for such examination, unless in the judgment of the Director, it is for the best interests of the service that original examination for such vacancy be held. In promotional examinations, efficiency and seniority in service shall form a part of such examination, but combined shall not carry a weight of more than 25% of the total examination points. Although efficiency and seniority in service shall not carry a weight of more than 25% of the total examination points, the Director may require candidates to separately pass the efficiency and seniority parts of the examination in order for the candidates to be eligible to take the subsequent parts of the examination. If the Director requires candidates to separately pass the efficiency and seniority parts of the examination, then any candidate who does not pass the efficiency and seniority parts of the examination shall fail the entire examination. All examinations for promotion shall be competitive. The method of examination, the rules governing the same, and the method of certifying shall be the same as provided for in the original examination.
70 ILCS 2605/4.10