While I want to applaud units for sending out congratulatory notices, I honestly wish they wouldn't. For exactly the reason you mention....it is hard to know what that means.
For each board that convenes, as they do their ranking, there are the selected list, and then there are a few that were close enough that they are placed in a bin of alternates. Once in a while, especially on the early boards, applicants drop out for a number of reasons. Maybe they were offered a USNA appointment, or a different SA, or maybe they had a medical issue and are no longer competitive, or they really only wanted 1 college and got rejected, so they are going to wait for next year. Anyway, when they make their decision early enough that they won't accept the scholarship offer, it creates an opening.
As those openings increase, we look at the alternate pool to potentially fill them. But, we have to gather up all the alternates and see what their scores are, and then rack and stack those. So the truth is, the alternates will be waiting for quite a bit before hearing back on anything. And there is no way to know where you fall in the list of alternates.
What I can say is that if a person made it to the alternate list, then their record was very competitive. And they have a high likelihood at getting the 3 year sideload even if they don't get the 4 year offer.