1. Is the promotion given to the employee after the interview or is just promoted without any interview.

2. Assuming that for promoting to the post of CAO, the feeder posts are SAO and SSO, an interview was conducted then the order of merit has to be followed in terms of seniority. In other words,  an employee who has scored more will be placed in the first position and so on.

3. In case there will be no interview and promotion is effected, then the date of joining in the feeder post will be considered for preparing seniority list. However if two people join on the same day for a particular post in the same organisation, then the person who is older in age has to be considered as senior.

It will depend on central government service rules and policies

Then you can file a writ petition in high court and it will decide the same in light of supreme Court judgement.  May be your case gives rise a some guidelines in the same

The date of appointment will only determine the seniority and nothing else

Share the copy of the CAT order

Every post in any of the department has its own qualifications and experience both individually some post has to be filled up by departmental candidates who fulfill the desired elements and some from the out side candidates, now if you segregate between two posts this will against the principle of natural justice to the post mentioned so if Parker's law has been followed then no need of worry that is sufficient for the purpose. 

1. As per the information mentioned in the present query, makes it clear that there is no ground to consider the date of process started rather than the date of appointment.
2. It is the appointment which states the criteria for knowing the seniority in the long run.
3. The aggrieved party should fuel their reply if the case has been filed it approach the Hon’ble court of appeal court to state that the stands are arbitrary and unconstitutional and also challenge law the ruling as no presence of any ruling with respect to the same issue.

Dear 

In this case the seniority is determined between the eligible candidates by the date of joining on the above mentioned cadres from where the promotion is going to happen.

If date of joining clashes then it will be determined by education and in last by age. 

The appeal will not help the department much as it doesn't matter the person is directly recruited to the post or by promotion.