It's a style that's applied to elements that have a container <div>. Use it to style the container or target and style any child elements.
The layout custom styles appears in the Custom style drop-downs for elements that are a container or have a containing div element. This includes Columns, Containers, Slider containers and the WYSIWYG element. and not a specific element.
You can use the Layout custom style to style the container and to style any child elements of the container using Child selectors.