The CSS class field can be used to make a block visible or hidden based on a device's view size (desktop, tablet, phone). This is useful for large blocks (like a slideshow, a leaderboard ad block, etc.) where they won't fit width-wise on a phone, so you can use a CSS class to make the block visible on large sizes, or hide the block on small sizes.  

appearance tab css class field

The table below lists the available visibility classes supported by Bootstrap.

Note: To include multiple classes, use a space between them. The below example illustrates how to hide a block for medium and small view sizes. 

Note:  Do not use leading characters or trailing characters, only the CSS Class Name itself.


CSS Visibility Classes

More information on Bootstrap CSS: