As stipulated in Section 16-166, the Director of Human Resources Management shall derive each certification list from the eligibility register established to fill a vacant position or positions in a particular class of work. However, if at the time of personnel requisition to fill a position the appropriate eligibility register has not been established or the appropriate eligibility register, although established, does not contain the names of a sufficient number of eligibles to permit certification therefrom, the Director of Human Resources Management may derive the necessary certification list from an alternative eligibility register. For purposes of this Section, an "alternative eligibility register" shall mean any eligibility register established to fill vacant positions in a comparable class of work with the same grade and equivalent qualification requirements or in classes of work with higher grades within the same class series.