In general, based on reports from admitted students posted here on CC over the past few years, it appears that merit aid goes to students in the top quartile of admitted students. Currently that means SAT 2200 and above, ACT 34 and above. The higher the test scores, the more merit aid. This assumes a comparably high GPA. Northeastern makes no secret of it that they are looking for high statted students. Selection for Scholars is a black box. In looking at the geographic distribution of Scholars they appear to be looking for geographic diversity both domestic and international:
https://www.google.com/maps/d/viewer?mid=zlqIr4NDmTcE.kGvcb6JVN4bo
Now none of the above explains why some 2300 SAT/3.8 UW GPA students are denied, waitlisted or offered NUIn. Reports on CC show that has happened.