First, you don’t even manage some state (like textures).
The framework handles those for you. So a more useful definition of
state is “whatever data you need in order to rebuild your UI at any
moment in time”. Second, the state that you do manage yourself can
be separated into two conceptual types: ephemeral state and app state.

Ephemeral state

Ephemeral state (sometimes called UI state or local state)
is the state you can neatly contain in a single widget.

This is, intentionally, a vague definition, so here are a few examples.

current page in a PageView

current progress of a complex animation

current selected tab in a BottomNavigationBar

Other parts of the widget tree seldom need to access this kind of state.
There is no need to serialize it, and it doesn’t change in complex ways.