Polygon is represented with the  icon.

Polygon exists for when you want to represent something impossible to do in Tableau. When you select Polygon, a new property is available: Path. To create a Polygon, you'll need the following:

• A Dimension representing the unique identifier of each Polygon 
• A Number in Path to connect the points and draw the shape of the Polygon
• One Continuous field in rows and another one in columns to generate the coordinates

Once you have that, you can create, for example, this:

In this screenshot, you can look at the average seat price in a stadium thanks to Polygons. This visualization comes from a tutorial by Ryan Sleeper available in his website: https://www.ryansleeper.com/how-to-make-a-custom-polygon-map-in-tableau.

For a long time, Polygons were the only solution to create this kind of visualization. Recently, Tableau introduced the shapefile connector to render spatial objects. However, a shapefile is fixed, and the coordinates of the Polygons aren't. That's why, to create flow diagrams such as a Sankey, the Polygon remains useful.

Let's finish with the most recent addition in the Mark type: Density.