25 (1) This section applies if a new bargaining unit is established or the description of a bargaining unit is changed by an agreement under section 20 or an order under section 22 and a collective agreement containing seniority provisions applies to any employees in the new bargaining unit or the bargaining unit whose description is changed.
 If one collective agreement applies (2) If only one collective agreement containing seniority provisions applies to any employees in the bargaining unit, the seniority provisions in it apply to all employees in the bargaining unit. If two or more agreements apply (3) If two or more collective agreements containing seniority provisions apply to employees in the bargaining unit the following apply: 1.  If the bargaining agent representing the employees in the bargaining unit was a party to one of the collective agreements immediately before the changeover date, the seniority provisions contained in that collective agreement apply to all employees in the bargaining unit. 2.  If the bargaining agent representing the employees in the bargaining unit was a party to more than one of the collective agreements immediately before the changeover date, the employer and the bargaining agent may agree as to which collective agreement’s seniority provisions will apply to the employees in the bargaining unit or, if they do not agree, either of them may apply to the Board for an order determining which collective agreement’s seniority provisions will apply to the employees in the bargaining unit. New bargaining agent (4) If two or more collective agreements containing seniority provisions apply to employees in the bargaining unit but the bargaining agent representing the employees in the bargaining unit was not a party to any of them immediately before the changeover date, the employer and the bargaining agent may agree as to which collective agreement’s seniority provisions will apply to the employees in the bargaining unit or, if they do not agree, either of them may apply to the Board for an order determining which collective agreement’s seniority provisions will apply to the employees in the bargaining unit. Application of subs. 33 (3), (4) (5) Subsections 33 (3) and (4) apply, with necessary modifications, to all employees in the bargaining unit. Other seniority provisions voided (6) If a seniority provision that would not apply to an employee in the bargaining unit under section 24 is made applicable to an employee under this section, any seniority provision in the collective agreement, if any, that would apply to the employee under section 24 or in the employee’s contract of employment is void. Amendment by parties (7) The seniority provisions of a collective agreement may be amended by agreement between the bargaining agent for the bargaining unit and the employer subject to the following: 1.  Despite any such amendment subsections 33 (3) and (4), as applicable under subsection (5), still apply. 2.  The provisions as amended must provide that, to the extent possible, the seniority of employees is based solely on a common definition of seniority and determined with reference to the bargaining unit as a whole and not to only a part or parts of it. Amendment by Board (8) The Board may, upon application of the employer or the bargaining agent for the bargaining unit, amend the seniority provisions of a collective agreement unless the provisions apply, under section 24, to all the employees in the bargaining unit subject to the following: 1.  Despite any such amendment subsections 33 (3) and (4), as applicable under subsection (5), still apply. 2.  Unless the Board considers it inappropriate in the circumstances, the provisions as amended must provide that, to the extent possible, the seniority of employees is based solely on a common definition of seniority and determined with reference to the bargaining unit as a whole and not to only a part or parts of it. Certain other provisions (9) If the seniority provisions of a collective agreement are made applicable to all employees in a bargaining unit under this section, the following apply: 1.  The provisions of that collective agreement respecting the posting of vacancies and new positions, promotions, transfers of employees, lay-offs and recalls also apply to the employees in the bargaining unit. 2.  Any other provisions of that collective agreement that the employer and bargaining agent agree should apply, also apply to the employees in the bargaining unit. 3.  Subsections (7) and (8) apply, with necessary modifications, to provisions applicable under paragraph 1 or 2. Section ceases to apply (10) This section ceases to apply when the first collective agreement is made after notice to bargain is given in the following circumstances: 1.  A party gives notice to bargain under this Act. 2.  A party to a collective agreement continued under subsection 24 (2), a composite agreement or a replacement agreement gives notice to bargain under subsection 47 (2) of the Fire Protection and Prevention Act, 1997 or section 59 of the Labour Relations Act, 1995. Definition (11) In this section, “seniority provisions” means provisions that give employees rights that depend upon their seniority.  1997, c. 21, Sched. B, s. 25.