The national pool is used to fill the class...since all the nominating/charging sources do not typically add up to the needed class size.
The bottom line from the national pool....AT LEAST 75% must have a nomination made by a public official in paragraphs (a)(2)-(8) (https://www.law.cornell.edu/uscode/text/10/6954). Only up to (could be less) 25% can come from some other source. This is why BGOs and USNA Admissions STRESS to apply to ALL nominating source (including MOCs/VP).
As an example, if a candidate only has a Presidential nomination and is not one of the 100 charged to the President, then they are already significantly disadvantaged in the national pool --- unless they are an extremely competitive candidate (which is very hard to know). If the same candidate has a MOC and Presidential...then there is no issue...since they can be picked in either scenario.
As time2 posted...plenty of more "detailed" threads on the topic.