"confirmed Seat"

Hello,
What exactly means : needs to possess an onward ticket with "confirmed date & seat" ?
I will be doing Sydney>Shanghai>Paris>Beijing>Sydney
to leave Shanghai after my 100hours transit, i have an e-ticket with Qantas / Air France with confirmed dates but not "confirmed seats number" ,
Does that means i need to check-in all the legs (all the way to Paris) when first departing at Sydney Airport ?
Thx for your opinions
Regards
Laurent