Every professional in FEL theory knows that in 40 years since its development, this branch of science has become a boundless ocean of knowledge and information. But, not having a system of dependable beacons, one can easily get lost or even drown in this ocean. Therefore, a demanding question arises for any author who aims to write this type of book and that is the question of what beacons he intends to use. This author was not exempt from this question. As mentioned in Chapters 1 and 9, one of the basic beacons is the phrase “cluster high-current FEL.” These words fully represent the basic trends of this book as a whole, as well as this and the next chapter, which are dedicated to the theory of the parametrical (ordinary) FELs.