Extreme formations were line-ups that abandon one part of the field.
As a result of season 42 match engine changes, this concept became obsolete.
A change in the rules
After 2010 reform, only three line-ups can still be played, and now also gain routine:
Other more unusual and drastic formations not listed above, occasionally used particularly by national teams, are no longer possible after the ban on more than three central defenders, inner midfielders, or forwards.
When playing 2-6-2 very experienced players are needed to prevent confusion. As you can achieve almost identical results with 3-5-2 and Offensive Central Defenders without confusion this tactic isn't recommended to the club teams unless the manager is certain what he is doing.
Normally this tactic was used only national teams, who can't suffer the training loss since they do not train players. But there were also clubs that used this line-up in a very important match; they chose to to lose training for a good result.
This is an example of 2-6-2 where is tried to dominate the midfield and also boost the attack ratings as much as possible. The defense will be very poor but a better midfield reduces the number of chances that the opponent will get. And with nice attack ratings the team will possibly score a lot of goals.
In this line-up the sole goal is to boost the midfield as much as possible, in order to receive as much chances as possible and prevent the opponent to get any chances. All 4 inner midfielders, the two wingers AND the two offensive central defenders are selected on their Playmaking and Stamina, thus the midfield rating will be maximised. The forwards will get a lot of chances and they will have to knock some in because the defence is actually formed by just a goalkeeper, so if the opponent has a chance it will probably go in.
The 2-7-1 (also known as the "Montgomery Formation") was a very midfield-heavy line-up, using at least 5 inner midfielders. It was usually undesirable for clubs because fielding more than three inner midfielders it caused training loss. National teams sometimes used this formation to gain a superior midfield, but it certainly caused confusion as it was impossible to gain routine.