The board shall make and keep a register of all persons eligible and available for appointment to each class of position in the service of the city, ranked according to ability; it is provided, however, that no examination shall be given and no register kept for positions to be filled by persons designated by the board as common laborers. Layoffs available for reemployment shall be placed at the head of the proper present and subsequent eligible registers in the inverse order of their terminations. Employees who voluntarily terminate their services may be granted reemployment status upon proper eligible registers under such circumstances and in such manner as may be provided for the board's rules and regulations, subject, however, to stipulations of this section concerning layoffs. Persons desiring appointment may file applications with the board, and the board shall, from time to time, conduct examinations to test the ability of such applicants. All qualified applicants shall be subject to limitations specified by the board as to age, residence, health, height, weight, habits, moral character, and other factors pertinent to ability to discharge the duties of the position, open to all citizens of the United States. Examinations shall be practical in character and shall relate to those matters which test the ability of the person examined to discharge intelligently the duties of the position for which he or she applies. In no case shall an appointment be made from an eligible register which is more than two years old, and no eligible register shall be the result of more than one examination.