national pool

This question may have already been asked, and if so, redirection to the thread would be great, but what is a nomination from the “Secretary of the Navy”? And how does the “national pool” of candidates work?

My turn to ask a question (I don't think I have ever done it on this forum)....this is triggered by an recent exchange of private messages with a parent, and numerous vague references to the "National Pool" in this forum. The term usually comes up when someone refers to not winning their MOC...

I have noticed some talk of a national pool. Could someone explain what it is, or direct me to someone who has?