Extreme formations are those formations that cannot retain routine from match to match. As a result, confusion events with these formations is quite common. The formations are considered to be extreme because they usually abandon one part of the field.
The three most common Extreme formations are:
These three formations, and only these, will not sacrifice training for the week as long as they are played with at most three players at each position.
The squad using these formations must have a high experience level to counteract the confusion created by using a formation that is virtually new to the team, even if you use it every week, since it is impossible to raise the experience level of the formation. One notable exception is with 5-2-3 with two wingers and counterattack tactic; such teams don't want any possession at all, solely relying on their counterattacking ability to score. In this case the team wouldn't really suffer any ill effects of confusion, since midfield ratings don't matter.
Those listed above are the most common, but there are numerous other extreme lineups which have been used over the years such as 2-6-2 and 2-7-1. Please beware however that there is no way you can play any other Extreme formation than these three and still keep your weekly training.