first ms ad stat@FSU

第一个AD，DIY申请的，有个问题不是很清楚
邮件里说 I would like to make an admission-only offer (without financial support) to each of you. If you are interested in this offer, please reply to me before April 15 and I will send you a formal offer letter.
这个是我决定去FSU的时候才回复interested并拿formal letter，还是我先说我先表示感谢和感兴趣把formal letter拿了再决定去不去？
这个formal letter到底是什么？