Usage of the CrossSection modifier is by and large determined by the arrangement of the splines to which it is applied. For details, see the parent of this topic.
Select a spline object with spline cross sections. > Modify panel > Modifier List > CrossSection
Default menu: Make a selection. > Modifiers menu > Patch/Spline Editing > Cross Section
Alt menu: Make a selection. > Modifiers menu > Spline > Cross Section
Linear/Smooth/Bezier/Bezier Corner
Determines what type of curve will be used through the spline vertices.