The nomination letter is intended to provide external recognition supporting why the candidate is eligible and should be strongly considered for the award.
Its purpose is to make an objective case for the significance, visibility, and impact of the candidate's scientific contributions within the field and broader community.
The reference letters serve a somewhat different role.
Referees are typically individuals who know the candidate, their work, and their trajectory more directly.
In addition to commenting on achievements, they are expected to provide perspective on the candidate's leadership, independence, future potential, and anticipated long-term impact on the field.