The personnel director shall keep a register of all persons eligible and available for appointment to each class and position in the service of the county under this part, ranked according to ability. Employees laid off who are subsequently available for reemployment shall be placed at the head of this list for eligible registrants in the inverse order of their terminations. Employees who voluntarily terminate their services may be granted reemployment status only under such circumstances and in such manner as may be provided for in the board’s rules and regulations.