When you get the bookings for the flight done in China eastern airlines, then you have the option of China eastern airlines seat selection that is provided to you when you get the online check-in done.

But that is only after China eastern ticket verification is done. As the seat selection advantage is only provided to the ticket holders who have booked the flight tickets via the official website.

So, only if you do not have an economy class ticket and the booking for the same was done via the official website is when you can get the China eastern airlines seat assignment.