Key crop rotation has it's disadvantages.  Sometimes vegetables from the same vegetable group are planted consecutively the same bed, though this is fairly rare.  It also means that the planting of leafy greens, fruits, roots and then legumes in a sequential order is not strictly followed. However the big advantage of key crop rotation is that it is more flexible and a lot simpler to administer.  You don't tie yourself in knots trying to keep the various crop groups separated.

The key crops are not fixed, it all depends on what vegetables you grow that take up the most space.  Nor do they all have to come from different vegetable groups, though that is preferable. 

My four key crops are tomatoes, corn, onions & carrots and broad beans & peas.  I rotate these four groups of vegetables across five beds of varying sizes, planting all my other vegetable varieties that I grow in whatever space is left after the key crops have been planted. 

But whatever crop rotation you are practicing don’t forget to record what was planted where in each season.  If you do that you will find it much easier to follow your particular plan.