求助！在eshop.cd.cz买票遇到的问题！

准备在这个新年，从德国==》维也纳==》布拉格==》德国，自助旅游一圈。在网上看到有朋友推荐从CD的网站购买电子车票比较便宜。于是，准备购买维也纳==》布拉格的单程车票；但是，在购买的过程中看到下面这样一段话，似乎要求同时出示维也纳和布拉格之间的往返票，才能有效？！哪位熟悉情况的朋友解释一下？

The use of an eTiket document from Austria to the Czech Republic is contingent on the purchase of an eTiket document from the Czech Republic to Austria and both eTiket documents must be presented for inspection to Czech Railways train personnel on the journey from the Czech Republic. eTiket documents from Austria to the Czech Republic purchased separately and eTiket documents not stamped by Czech Railways train personnel are invalid! If needed, it may be necessary to actively search for Czech Railways train personnel and have your eTiket document stamped.