The subject line is the introduction or greeting of your message.

引进或问候您的邮件的主题行。唉呀！

youdao

Just select one of the pre-defined subject headings and type in your enquiry or message .

选择一个预先定义的标题，再键入您要查询的内容或留言即可。

youdao

Please include the title of theme in the subject line of your message and remember to add your name and a caption: who, what, where and when should be enough, though the more details you give, the better your chance of being selected.

BBC: We set the theme; you take the pictures