For the first time at a World Cup, FIFA is using head-to-head records instead of group goal difference as the primary tiebreaker for teams level on points.
Until 1966, goal ratio was used to separate teams - this involved dividing a team's goals scored by goals conceded. In 1970, FIFA moved to goal difference - until now.
It brings FIFA in line with UEFA, which has always given priority to the results between teams.
The logic is that assessing the direct performance of two teams is fairer, because it removes freak results - such as Germany's 7-1 win over Curacao.
Others say that taking overall goal difference is better as that compares the total record in the group.
FIFA first introduced it at last year's Club World Cup, where Flamengo won Group D ahead of eventual tournament winners Chelsea after two matches.
Here's how it changes things at this World Cup.
Why Mexico know they have won the group
The main knock-on effect is a greater chance of teams not only advancing but winning the group before matchday three.
Under the old format, a team would need to be four points clear of second to lock top spot after two games.
That was rare because it required the other two fixtures of the first two rounds in the group to both be draws.
Now it is possible to be three points clear and get first place in the bag.
It can happen a few ways.
It requires either two draws, or first place to have beaten the team(s) on three points.
We have already seen it in Group A.
Mexico won their first two games and sit on six points - three points clear of South Korea, with the Czech Republic and South Africa on one point.
As Mexico have already beaten South Korea they cannot be overtaken if both countries end on six points. So Mexico have won the group.
They know they will play a last-32 tie in Mexico City against a third-placed team.
But there's a knock-on effect.
Mexico effectively have nothing to play for when they meet the Czech Republic next Wednesday, and could opt to rest players.
And that creates an imbalance the Czech Republic could benefit from, though they can only qualify automatically if South Korea lose to South Africa.
But in a tournament where the best third-placed teams go through, it could be important.
The system is also used at the European Championship, where the best third-placed teams advanced.
Who else could win their group on matchday two?
For top spot to be locked after round two, a few things need to be considered:
The top team must win both their games
The two other fixtures must have draws, OR
First place must have beaten the team or teams on three points
GROUP C
Scotland will qualify as group winners if they beat Morocco and Brazil do not beat Haiti.
GROUP D
The United States take top spot with a win over Australia as long as Turkey do not beat Paraguay.
Australia can do so with a victory if Paraguay do not win against Turkey.
GROUP E
Germany win the group with a victory over Ivory Coast if Ecuador do not win against Curacao.
Ivory Coast will seal top spot with a win if Curacao do not beat Ecuador.
GROUP F
Sweden will qualify as group winners if they beat the Netherlands and Japan do not win against Tunisia.
GROUP G
No team can seal top spot on matchday two.
GROUP H
No team can seal top spot on matchday two.
GROUP I
No team can seal top spot on matchday two.
GROUP J
Argentina take top spot with a win over Austria as long as Jordan do not beat Algeria.
Austria can do so with a victory if Algeria do not win against Jordan.
GROUP K
Colombia will seal first place if they beat DR Congo and Portugal do not win against Uzbekistan.
GROUP L
England win the group with a victory over Ghana if Panama do not win against Croatia.
