The set of models that are pareto-optimal, IE for some set of variables, no other model strictly dominates them = no other model is better than them on every variable.

So like, on a cost-intelligence graph, the cheapest and most intelligent models are pareto optimal. Then in-between those if you have

- cost $3 intelligence 6

- cost $1 intelligence 5

- cost $2 intelligence 4

The 1st and 2nd are pareto optimal, the 3rd is not, because it's dominated by the 2nd (2nd is cheaper AND more intelligent at the same time)