Looks like it’s strength of victory in the NFL, and it’s more complicated than just point differential, but i’m guessing RB just goes with PD. Per NFL rules though: “To determine the best combined ranking among conference teams in points scored and points allowed, add a team’s position in the two categories; the lowest score wins. For example, if Team A is first in points scored and second in points allowed, its combined ranking is “3.” If Team B is third in points scored and first in points allowed, its combined ranking is “4.” Team A then wins the tiebreaker. If two teams are tied for a position, both teams are awarded the ranking as if they held it solely. For example, if Team A and Team B are tied for first in points scored, each team is assigned a ranking of “1” in that category, and if Team C is third, its ranking will still be “3.” “