A bargaining unit is a group of employees who are represented by a labor union in their dealings with agency management. Bargaining unit status pertains solely to the positions employees hold within the agency. Employees are either in a bargaining unit (bargaining unit members) or are unrepresented.
Employees may elect to join the union and pay union dues. Such employees are union members in addition to being members of the bargaining unit. A union that has been accorded representational rights over a bargaining unit must represent all employees covered by that bargaining unit, regardless of whether or not they pay union dues. This is known as the duty of fair representation.
Certain employees are excluded from bargaining unit coverage (e.g., supervisors, management officials, confidential employees with respect to labor relations matters, personnelists in non-clerical positions, and certain individuals employed in intelligence, counterintelligence, audit, investigative, and security work or functions).
Most DoD activities or installations have at least one bargaining unit. However, some activities or installations have more than one bargaining unit.
REF: Title 5 USC Ch 71; 5 CFR Chapter XIV; Civilian Personnel Manual Subchapter 711, "Labor-Management Relations"