Xsections are global objects. As such you do not have to add them to cells as a xsection before you can use them. You can just use them when they are defined, see xsections and layers.
Note that Polygon objects, as used in the example you refer to, do not work with xsections. They work with layers.
Conceptually a xsection is mostly applied to represent a waveguide or metal guide, consisting of one or more layers, captured in a xsection. Polygons are more free form geometries for drawing, like when used in a BB as in the example.