For clarity, I would add that when you say “Likely Letter”, to me that means a formal letter from one of the eight Ivy League schools. If that is what you mean, then @skieurope 's response is exactly correct. It is the equivalent of an acceptance letter.
Just remember that like any acceptance letter, it can be revoked if the student’s grades tumble, they are convicted in court, or do something that is embarrassing to the university (e.g., recent Harvard meme shenanigans). Having said that, few of them actually do get revoked.