Some tasks require you not just to calculate the Rotationsvolumen of a single function and the x-axis but the area between to functions when the graphs are "rotiert".
Those tasks might ask you to determine the volume of glass needed in order to form a vessel where f(x) is the inner function ("untere Schranke") and marks the area of where potential fluids flow in and where g(x) is the outer function ("obere Schranke") which marks the outside of the glass body.
The volume of the glass body would be the Rotationsvolumen that emerges from the area between the functions f and g.
Some tasks require you not just to calculate the Rotationsvolumen of a single function and the x-axis but the area between to functions when the graphs are "rotiert". Those tasks might ask you to determine the volume of glass needed in order to form a vessel where f(x) is the inner function ("untere Schranke") and marks the area of where potential fluids flow in and where g(x) is the outer function ("obere Schranke") which marks the outside of the glass body. The volume of the glass body would be the Rotationsvolumen that emerges from the area between the functions f and g.