Apple’s HIG:
> Show one popover at a time. Displaying multiple popovers clutters the interface and causes confusion. Never show a cascade or hierarchy of popovers, in which one emerges from another. If you need to show a new popover, close the open one first.
> Use submenus sparingly. Each submenu adds complexity to the interface and hides the items it contains.
> Limit the depth and length of submenus. It can be difficult for people to reveal multiple levels of hierarchical submenus
Thanks @valery_kirichenko for the video.