Is Seat Fee compulsory on JR Rapid trains?

Hi! Would like to know if we're riding the JR Rapid Airport train from CTS to Sapporo and there's a seat fee indicated according to Hyperdia. Can we opt not to choose a seat or is it compulsory to pay for an allocated seat? If there's a seat fee option, do we buy the tickets from a JR counter instead of a vending machine at CTS? Thanks for all help!

No, it is optional.

Some rapids have only reserved seats, in which case you have no other choice than to take one, and pay the additional couple hundred yen. It's mostly tourist trains, however, such as the Resort Shirakami or the SL Htoyoshi.

That option "310Y plus" means only for "U seat(uシート)" as seat reservation fee.
Other regular all seats(普通車) are "non-reseved" no need 310Y.
http://www.jrhokkaido.co.jp/network/airport/
https://www.jrhokkaido.co.jp/train/tr020_02.html
http://www2.jrhokkaido.co.jp/global/english/train/tr020_01.html
http://www2.jrhokkaido.co.jp/global/english/train/tr020_02.html
http://ameblo.jp/ohisan2424/entry-11933109623.html
http://sapporosabway.blog.fc2.com/blog-entry-94.html

@ Firas
Dont forget "i" SL H(i)toyoshi.
And meaningless JR-East and JR Kyushu trains as examle.

And meaningless JR-East and JR Kyushu trains as examle.

Don't forget the "p" at "exam(p)le".

Further, the title of this thread is "Is Seat Fee compulsory on JR Rapid trains?". Someone coming across this thread after a Google search could have concluded that they are alwys optional. In general, I feel answers should be as general as possible, to be useful for as many people as possible.

So can I conclude that seat fee is optional and if we were to take JR Rapid Airport 195 for OTARU train we can just get the tickets from the vending machines? Can we use the Kitaca card to just tap in? And if we wish to book a seat we should go to the counter frr seat reservations ? In this case cannot use Kitaca? How do we know if the train only takes reserved seats? Is this indicated in Hyperdia? We are easy on any types of trains, as long as there's seats. We would have flown 10hours that day and surely could use a seat! Thanks again!

In this case, yes, seat reservations are optional. A rapid train without a reserved seat is exactly the same as a local train for ticketing purposes: you can buy a ticket at a ticket machine or ticket counter, or use your IC card. If you want a reserved seat, you can either buy a separate reserved seat ticket, or a combined base fare + reserved seat ticket, at a ticket machine or ticket counter. If you buy a separate reserved seat ticket, you can use your IC card to pay the base fare (in this case you must insert the ticket first in the ticket gate, and then touch your card).

To know what seating options are available on a particular train, you can look at the available options in the drop-down list in the "Seat fee" column of Hyperdia.

Thanks so much, Firas!