Text added to the Joomla 3 module CSS class to make it unique via the Module CSS class suffix option, which allows you to create an individual design for the module. In Joomla 4, the option is called Module CSS class, and differs in that it specifies a new class for the module, rather than adding a suffix to the standard class.
This option is available on the module editing page in the Options tab.
Parent tag Joomla modules have a default class of moduletable:
<div class="moduletable">
Filling in the text area of the Module CSS class suffix will append the entered value to the class name. For example, if you enter the value mymodule, you will get:
<div class="moduletablemymodule">
To add a new class to a module in Joomla 3, the option value must begin with a space:
<div class="moduletable mymodule">