Usually page, but also chunks of the page... Same element for whole device. All the stuff we talk about, like size, bezel obscuring, indicator or interactive, relative size (suggested!) and so on. Space saving techniques like letting it overlay content, vs being a strip, and letting it disappear being not suggested (how to tell where you are without scroll, when eyes are somewhere else often).
Problem
Solution
Variations
Interaction Details
Presentation Details
Antipatterns