In Tableau, Shelves and Cards can hold fields. As you know, fields are the columns of data in a dataset.

Here I am using the Excel file Sample – Superstore. I am working with the Orders worksheet.

In the screenshot below I have dragged Sales to the Rows and Region to the Color. When you drag a continuous field, like Sales, to a row or column, Tableau creates an axis instead of a label. An axis is like a range of infinite values. When I dragged Region to Color i automatically got a legend at the right. The red text is something I added to the screenshot with Photoshop.

The Tableau website has information on shelves and cards.