It may be helpful to go read the sticky posts pinned to the top of the Nominations forum, in particular the link to the Congressional Guide.
Broadly speaking, other fully qualified applicants on an elected official’s slate, if not chosen as the candidate being offered the appointment being charged to the elected official for that admissions cycle, may also be offered an appointment following certain guidelines - their appointments are charged to a nominating authority managed by the SA. Those have-a-nom-and-fully-qualified candidates essentially compete nationally against each other for those other possible appointments. I don’t think you will find “national pool” on SA websites, but I haven’t looked recently just for that.
If you dive into the Congressional Guide, it explains all the nominating authorities and how many appointments can be charged against each.