Areas of existence of ruled surfaces

Ruled surfaces are used in mechanical engineering, in aircraft and motor-vehicle constructions, in agriculture, in construction and light industries. It is easier to denominate areas where ruled surfaces are not applied than to list them all. The value of ruled surfaces in human activities can not be overestimated. In this paper the consideration of the formation of ruled surfaces with a uniform way of setting them, which was proposed earlier in two articles with the same title "General Principles of Defining Ruled Surfaces", is continued. The concept of a limit ruled surface is introduced in this article, and on the basis of a single method for defining ruled surfaces, the question of identifying the areas of their existence that lie between two limiting ruled surfaces is considered. These areas of existence are, in fact, a line congruence.