In this subsection the following words have the meanings indicated. “Confidential employee” means an employee who acts in a confidential capacity with respect to an individual who formulates, determines, or implements management policies in the field of labor–management relations. “Probationary employee” means a merit system employee during the employee’s initial probationary period following employment. The rights granted to Commission merit system employees under this subtitle do not apply to: attorneys in the General Counsel’s office; confidential employees; employees who are at grade J or above; park police officers; the Inspector General; and any subordinate employee of the Office of the Inspector General appointed under § 15–503 of this article; probationary employees; or supervisors, as defined in § 2(11) of the National Labor Relations Act, 29 U.S.C. § 152(11). Commission employees are divided into four bargaining units consisting of: the office unit that includes office classification titles in which employees are responsible for internal and external communications, recording and retrieving information, and paperwork required in an office; the professional/technical unit that includes: professional classification titles in which employees have special or theoretical knowledge that usually is acquired through college training, other training that provides comparable knowledge, or work experience; paraprofessional classification titles in which employees perform, in a supportive role, some of the duties of a professional or technician but that usually require less formal training or experience than those duties performed by those with professional or technical status; and technical classification titles in which employees have a combination of basic scientific or technical knowledge and manual skill that usually are acquired through specialized postsecondary school education or through equivalent on–the–job training; the service/labor unit that includes classification titles in which employees perform service and maintenance, may operate specialized machinery or heavy equipment, and whose duties contribute to the comfort and convenience of the public or to the upkeep and care of Commission buildings, facilities, or grounds; and the trade unit that includes classification titles in which employees are required to have a special manual skill and a thorough knowledge of processes that are acquired through on–the–job training, experience, apprenticeship, or other formal training programs. A bargaining unit may not be defined by county boundaries. If an employee organization is certified to represent more than one bargaining unit, the Commission shall negotiate a single contract with that organization covering all employees the organization represents.