关于西班牙ALSA汽车订特价票的问题

购买的西班牙从塞尔维亚去格拉纳达的特价汽车票，不能退不能改的那种，买的时候没说需要什么凭证，买好后发来的邮件里面说了这么一句“For the use of this type of ticket it is essential that the traveller have the valid document that shows he/she is the beneficiary of this discount. At the time of entering the bus, the traveller must show the aforementioned document to ALSA personnel. In case of not complying with this requisite, the driver has the obligation to impede the use of the ticket and in no case can the economic difference be paid, having to acquire another complete ticket if he/she wishes to travel”，似乎是我要出示什么享受这种折扣的证件啊？好奇怪啊，求有经验的网友么帮帮忙！