In addition to appointment or TWE, hearing something by April 15 can also mean being put on the wait list. DS was wait listed April 15 and got a TWE on June 4. DD was wait listed April 15, and received TWE on May 22nd. Both re-applied and received appointments the following January.
What does a wait list notice look like? Is it an email and portal "You've been wait listed"? Apparently wait-listing increases chances for acceptance next year?
Yes, it is an email and in the portal. You also have to respond to USNA telling them that you actually wish to be on the wait list.
As for reapplying and increasing your chances next year-- I've heard several times that being on the wait list means from admissions perspective: "you are qualified, we'd like to have you, but we just don't have room for you this year." How much of that is true, I really don't know. However, it makes sense that being wait listed is better than getting an immediate TWE. It shows admissions still has some interest in case a slot becomes available (even though very few if any have been appoint from the wait list in recent classes) So, if admissions is interested enough in a candidate to wait list him/her, instead of an outright rejection, it would make some sense that the board would like to see that same candidate reapply. Adding a year of good college grades taking "plebe like classes", improving test scores (including CFA), and staying out of trouble, should make you an even better candidate-- and hopefully increase chances for acceptance a second time around. Worked for my two Mids.