Selects the element that initiated the navigation, whether it's a link/button/form. The element stays selected throughout the navigation.
The main motivation for this comes from view transitions. This gives author a way to visually indicate a relationship between the navigation source (form/button) and the process of the navigation, e.g. expanding a thumbnail using an animation.
Explainers: https://github.com/WICG/declarative-partial-updates/blob/main/route-matching-explainer.md#link-matching