It's more commonly called the "National Pool," not to be confused with the "Wait List." The former are those nominees who do not "win" their slate. IOW, USNA selects one person from the slate of up to 10 who receives that MOC's nomination. If more than one person was nominated for that vacancy, the others go to the National Pool, where they compete against other non-slate-winners for additional offers of appointment.
The Wait List is made up of candidates who do not initially receive an appointment or a turndown. They are waitlisted, and depending on yield (acceptance rate), will later be offered an appointment or will later receive a turndown. The Wait List is typically not formed until very late in the admissions process.