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, as long as they are played with at most three players at each position, will not sacrifice training for the week. However, 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.
A useful rule of thumb for remembering which formations are extreme and which are normal is that when setting a line-up, if the only way to play the formation is by moving two or more players, then it is extreme. If the formation can be played by moving only one player, it is normal. However, this does not mean that you can only reposition one player without risking a confusion event. As long as the lineup can be created by repositioning only one player, you are not risking a confusion event (unless you have low experience with that formation).
Any lineup that requires repositioning two or more players is an extreme lineup. Those listed above are the most common, but there are also countless other extreme lineups not listed (such as 2-6-2 and 2-7-1) simply because it would take too much time to list them all.