The board shall, as soon as practical after this part becomes operative, adopt a classification plan and make rules for its administration. The position classification plan may, if desired, create different classes of positions within each position in the classified service. The position classification plan shall show the duties, authorities, responsibilities and character of work required of each position and each class thereof. The board shall determine the requirements of each position and class thereof as to education, experience, capabilities, knowledge and skill. As far as practical, the probable lines of promotion to and from the classes of positions shall be indicated.