Except as otherwise provided by law, this subtitle only applies to skilled service and professional service employees in the State Personnel Management System. The procedures in this subtitle do not apply to: a disciplinary action under Subtitle 1 of this title; a termination or separation from employment under Subtitle 3 of this title; or special appointees. This subtitle does not prevent the layoff of an employee who submits to the head of the employee's principal unit a written request to be laid off.