A common use case is an audio or camera indicator in a web browser.
adw.tab_bar.TabBar will show it at the beginning of the tab, alongside icon representing adw.tab_page.TabPage.icon or loading spinner.
If the page is pinned, the indicator will be shown instead of icon or spinner.
adw.tab_overview.TabOverview will show it at the at the top part of the thumbnail.
adw.tab_page.TabPage.indicatorTooltip can be used to set the tooltip on the indicator icon.
If adw.tab_page.TabPage.indicatorActivatable is set to TRUE, the indicator icon can act as a button.