With the migration to wlr_scene, some objects (desktop, output etc)
will no longer have any reason to care about individual shells and
similar; handling each one on its own would introduce code that is,
from their perspective, over-complicated.
Strata (sg. stratum; this name was chosen to avoid confusion with the
over-used 'layer') represent logical layers of surfaces on the desktop,
such as every layer shell layer, one for xdg shell, another for popups.
