Headings
Headings help to provide a hierarchy for the screen's content, and allow people using a screen reader to quickly jump between sections. Numbered headings (H1, H2 etc) are not supported by iOS or Android. Headings are the backbone of the content of a page.
A user should be able to scan the page using headings to get a good impression of its content. Headings aren’t just for visual hierarchy.
They also provide semantic — or meaning — hierarchy. Headings provide structure to an app, aiding in VoiceOver navigation and supporting neurodiverse users.
Navigation bar (usage)
For iOS and Android, it’s a boolean, you just need to mark whether or not something is a heading. There are no nested levels. The first heading preferably starts just above the main content. Do not choose a heading by its size, but by its level in the context of the content, e.g. navigation bar title describes what the page is about.
Recommended: Navigation bar's title should always be marked as heading.
Subheader (usage)
We recommend to use headings also for List Subheader component as its title usually describes the content below.