Why didn't I get a coach # and seat # with my Reservation ?

I booked second class tickets online from Le Havre to Paris and Paris to Le Havre on same day, one in the morning and one returning in the afternoon.
My trip from Le Havre to Paris (intercites 3108) gave me a coach# and seat #, but on the return trip Paris to Le Havre on (intercites #3121), It did not give me a coach # or seat #. My question is do I have reserved seats for the Paris to Le Havre trip(trip2) ?
If I do not have a seat, then do I get a seat or do I have to stand for the trip?
If I want a reserved seat what do I have to do?

Dave G.
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