Two Rules Are Following In A Company

In our company for departmental selection, two types of rules are follows. 1. For departmental selection from non-executive to executive they have done selection on the basis of merit and also their seniority in the selected cadre and grade is decided on the basis of merit. 2. For departmental selection from non-executive to non-executive in different cadre and grade they have done selection on the basis of merit but their seniority in the selected cadre and grade is decided by their joining in the present grade. Now my question is 1. Is seniority on the basis date of joining is an equal opportunity to all employees from a different establishment. 2. When selection is done from pure merit why seniority is done by date of joining. 3. iI a case can stand against it and what is the winning percentage.