g-menu-item-set-section (item section)

item -- a g-menu-item object
section -- a g-menu-model, or nil

Sets or unsets the "section" link of the menu item to section.

The effect of having one menu appear as a section of another is exactly as it sounds: the items from section become a direct part of the menu that item is added to. See the function g-menu-item-new-section for more information about what it means for a menu item to be a section.