Slideout menu is additional menu area slide from the side of the screen when you click or hover on 3 lines icon in header. Slideout menu will appear from the left if icon is in the left header section and from the right if in the right side of the header.
Slideout menu only displayed on desktop. On mobile header and all menus replaced with mobile header and menu.
To add slideout menu you first of all need to add its element in header.
Then create menu and select Display location :
Additionally you can add content at the bottom (i.e. social icons shortcode or copyright address info, etc. ) of the slideout menu area in Header – Header Layout section of 5th Avenue options panel.