This section does not apply to those units of State government with an independent personnel system. In the State Personnel Management System the Secretary shall: establish classes; assign a rate of pay to each class; ensure that each class comprises one or more positions: that are similar in their duties and responsibilities; that are similar in the general qualifications required to perform those duties and responsibilities; to which the same standards and, if required, tests of fitness can be applied; and to which the same rates of pay can be applied; give each class a descriptive classification title; prepare a description of each class; and create additional classes; and abolish, combine, or modify existing classes. The Secretary shall: assign a class to the skilled service, professional service, management service, or executive service, as appropriate; and designate special appointment positions in the State Personnel Management System that: must be filled without regard to political affiliation, belief, or opinion; or in accordance with § 6–405(b) of this article, may be filled with regard to political affiliation, belief, or opinion.