Display at top
A Topbar form appears at the top of the page in a banner style in an overlay. You can set the form to disappear when the user scrolls down the page as can be seen here. When you scroll down, Topbar form at the top will disappear. you can also close this form with the close button at the right corner of the form.
Display at the bottom
Topbar form can be displayed at the bottom of the screen as well. You can set it to display always and remove the option for the users to close this form as well. This will ensure the form is always displayed unless the visitor superscribes to your mailing list.
Why Topbar Form
Topbar form is an effective tool to have in your lead generation mix arsenal as it offers the flexibility to be always displayed without being intrusive.
With regards to how Google scores pages based on their accessibility on mobile devices, top-bar form is an effective workaround for using a popover form as a top-bar form is not considered interstitials and are not intrusive in any way. You can have your popover forms display only on desktop devices and use Topbar form only on mobile devices as an alternative.