You use re in documents such as business letters, e-mails, faxes and memos to introduce a subject or item which you are going to discuss or refer to in detail.
Dear Mrs Cox, Re: Homeowner's Insurance. We note from our files that we have not yet received your renewal instructions.
亲爱的考克斯夫人:关于房主保险一事,我们从自己的资料中注意到,我们还没有收到你续保的通知。