Square is represented with the  icon.

You can create two types of charts with Square: heatmap and treemap. 

A heatmap is a table, but better. When you build a table, you have to read every value to compare them. Heatmap gives the same level of granularity (the same amount of information), but with the ability to quickly spot the top and bottom values. Here's an example of a heatmap:

In the preceding screenshot, you can easily see that it's the Chairs and Phones of the Consumer segment that bring in the most money. 

A treemap is a hierarchical representation with Squares. The pros of a treemap are that it gives you a quick idea of the number of values and the proportion of each of them. Here's an example of a treemap:

In the previous screenshot, you can easily see the number of sub-categories and the best ones for each category.

The next Mark type, Circle, has a great advantage, which is very useful in data visualization. Continue to discover what it is!