From Joomla! Documentation
Page Class Suffix is a parameter in Joomla! content menu items. It is set in a particular menu item: [Edit Menu Item] page, under the "Page Display" tab. A new page class suffix will cause Joomla! to either add a new CSS class or modify the existing CSS class for elements in the assigned to this very menu item page layout.
When Joomla! generates a page, it automatically creates pre-defined CSS classes to allow styling of the page. For example, a page might have the element
To create a new class, enter the parameter with a leading space. For example, entering a space plus "myNewClass" will create a new CSS class called "myNewClass" and it will be inserted as a class for elements in that Menu Item. In this case the example above will be changed to
<div class="componentheading myNewClass">
To change the name of the existing class, enter in the parameter without a leading space. For example, entering "_mySuffix" (no leading space) will cause the HTML to changed to
Generally, it is recommended to use a leading space to create a new class. This way, CSS styling for this component that uses the standard class names will continue to work. You can use the new class name to add any desired styling to the component without needing to re-create all of the existing CSS code. Note that, if you create a new class name, make sure it has a unique name and doesn't conflict with any existing class names.