(b) The Board shall decide in each case, in order to assure public employees the fullest freedom in exercising the rights guaranteed by this Act, a unit appropriate for the purpose of collective bargaining, based upon but not limited to such factors as: historical pattern of recognition; community of interest including employee skills and functions; degree of functional integration; interchangeability and contact among employees; fragmentation of employee groups; common supervision, wages, hours and other working conditions of the employees involved; and the desires of the employees. For purposes of this subsection, fragmentation shall not be the sole or predominant factor used by the Board in determining an appropriate bargaining unit. Except with respect to non-State fire fighters and paramedics employed by fire departments and fire protection districts, non-State peace officers and peace officers in the Illinois State Police, a single bargaining unit determined by the Board may not include both supervisors and nonsupervisors, except for bargaining units in existence on the effective date of this Act. With respect to non-State fire fighters and paramedics employed by fire departments and fire protection districts, non-State peace officers and peace officers in the Illinois State Police, a single bargaining unit determined by the Board may not include both supervisors and nonsupervisors, except for bargaining units in existence on January 1, 1986 (the effective date of Public Act 84-1104) . In cases involving an historical pattern of recognition, and in cases where the employer has recognized the union as the sole and exclusive bargaining agent for a specified existing unit, the Board shall find the employees in the unit then represented by the union pursuant to the recognition to be the appropriate unit.
Notwithstanding the above factors, where the majority of public employees of a craft so decide, the Board shall designate such craft as a unit appropriate for the purposes of collective bargaining.
The Board shall not decide that any unit is appropriate if such unit includes both professional and nonprofessional employees, unless a majority of each group votes for inclusion in such unit.