Showroom > Main menu
Any section at your website can have its own main menu, or can just use main menu of parent section.
The very definition of the main menu of any section is done by simple drog&dropping any section from upper tree to the tree located at the left bottom corner of the screen (initially minimized region called Main menu of ...). It is however possible to click the Add button at this region as well and pick a section at the form shown afterwards. There you can also create menu link targetting URLs outside the system (just enter the URL instead of selection section).
Side note: Not that theres any restrictions regarding menus in Cloudia CMS, you can have any number of menus you'd like. Main menu is just a specific feature, as almost any website has one.