意铁上offer这一栏出现bimbi gratis，这是什么意思？

订单出现这个词，解释是：Solution description: Children (under 14 years old) free for groups from 2 to 5 people with at least one adult. Only one change and partial refund until departure. Subject to seat availability.最后一点是什么意思，需要补票吗？还是可能没有座位？