noun
reserved-seat ticket; seat reservation ticket
A ticket that guarantees a specific seat, commonly used for trains, planes, theaters, and events.
新幹線の指定券を買うのを忘れないでください。
Please don't forget to buy a reserved-seat ticket for the Shinkansen.
指定席 refers to the reserved seat itself, whereas 指定券 is the physical ticket or voucher for that seat.
Compound of 指定 (してい, 'designation, reservation') and 券 (けん, 'ticket').