The parent stack simply allows you to choose whether Popper stack will perform in either 'Framer' mode or 'Divider' mode.
If Divider mode is selected then you are given an additional option to specify the height of the divider. This value can be given using any appropriate units. The default value is 20px.
(As of v1.3) There is also the option to add a custom class. This could be useful (for example) if you want to set a height (using custom CSS) for Popper stack instead allowing the content to dictate the height.