I am not sure about that, many applicants on this forum received correspondence letters and still got rejected...Some I know wrote they got a correspondence letter before the medical request and came as a rejection,,,, so idk
There are different types of correspondence letters. One of the most common one is the biometrics correspondence letter in which an applicant should expect an answer ranging from a day to a week or even more....
Other correspondence letters can be like acknowledgement letters mentioning that we have received your app, etc etc..
As for the POE, it is the letter that states your application has been approved. Google it and see images or check this forum out...