When assigning Menus, Porto also offers several theme options to help customize the menu. When styling your various menu locations, the settings will be here. Some of these might be overrided by Porto Menu elements.
As you can see over feature, there are 6 types of main menu. And unders are the examples of 6 main menu types.
Normal: Maybe you can see normal main menu type in Home Classic.
Flat: Home Corporate 6 use flat main menu type.
Flat & Border: Shop Demo 30 use flat & border main menu type.
Top Border on hover: You can see this type in Home Corporate 8.
Thick Underline on hover: Please visit Home Classic Light and you can see this type.
Popup: This type is popular in business demos and you can see it in Architecture & Interior Design.
If you want to know about menu in more detail, please visit Menu Setup, How to build a megamenu and About the menu type.