When parental or student contribution (calculated according to the school specific formula) is deduced from the student's entire cost of attendance, the rest is called "unmet need". Schools that have money to spare for need based grant or scholarship will help their students with those unmet need, but only if the student first borrow some money themselves and this amount is termed unit loan.
A wealthier school or school with more budget dedicated to need based scholarship can afford to set the unit loan lower. For any students who has unmet need after EFC deduction it'll be good to have a low unit loan. This does not affect wealthier students who might have such a great EFC (which may not be justified in all cases, since not all children born to rich family will actually have the help from their parents, it's not like the school can force the parents to give the students money), they won't have any unmet need, so they just have to borrow. The amount of federal loan they are eligible to borrow are determined according to their parental income. If a student with exceptionally wealthy parents has a greater EFC than the cost of attendance, then at the end of FAFSA, the government might say that you are not eligible for any federal loan. (I need second opinion on the last point I made from
@breakintheroof @ProbablyAPenguin @hellanutella @onceawolverine )
Unit loan is composed of various type of loan and it's institution specific. At Yale, about $14k of the $27k unit loan comes in the form of alumni loan, about $4.5k comes as Perkins and another $8.5k as federal unsubsidized direct loan. Cornell has a similar breakdown except overall unit loan seems slightly higher. Some other schools I'm sure all unit loan are just federal loan. So for higher income student, it doesn't really matter the unit loan amount, what matters is the type of loan they are eligible to receive in that specific institution given their parental income and the cost of attendance
I must stop myself, but I really can go on and on about Fin Aid.