Extended probation - whether their seniority will be effected for the purpose of further promotion which is seniority-based? References?

Rule: recruited employees may be promoted to the next pay scale on completion of promotion (not vacancy-based promotion).
Proposition: if the probation of two employees were extended and were promoted to the next pay scale on completion of extended probation, whether their seniority will be effected for the purpose of further promotion which is seniority-based??? References ???

Dear Usman Haider,
You have asked for the rules related to the promotion of the employees. However, these are country-specific and if not specific to the country then specific to the company. There are no universal standards as such.
Please check your company's "Policy on Employee Promotion". If the policy says that to become eligible to be promoted to the next rank, an employee must complete "X" number of years/months after acquiring a status of "Confirmed Employee" then he/she is liable to serve for those many years/months. However, if no such rule exists then you may ignore their extension to the probation period and award them promotion like others.
Thanks,
Dinesh Divekar