Two-dimensional figures are one category and polygons are a separate category that falls under two-dimensional because 2D(two dimensional) figures are figures that do not have a volume so they are flat. Polygons are flat 2D shapes that have straight lines and all lines are closed, meaning that they don't have any disconnected lines. Polygons are a subcategory of 2D figures because they carry all traits of 2D figures but also have special ones of their own. Examples of polygons are squares, rectangles, and triangles. Examples of nonpolygons but still are 2D figures are circles, ovals, and any other flat shape.