Detail is represented with the  icon.

Like the previous Mark properties, dropping a Dimension on Detail splits the Marks. And that's it. The Detail property does nothing more than split the Marks. There are no different colors, sizes, or labels. 

You can use Detail to show your data at a less aggregated level. Here's an example: 

Tooltip is the next property, and it's the only one that doesn't split the Marks.