Umbra means shadow -but of course you knew that. I use the term umbratic in a sense I am not yet quite sure of but it is something approximate to the thing-in-itself. Yet here in this philosophy the thing in itself is a manifestation. For Kant it manifests as an unknowable [p]noumenon and Hegel it is the idea. A common modern conception is the absolute reality of the spatio-temporal framework. The universe continuing without our presence in the same manner. I’m not here to argue with this, I’m saying this is a manifestation of how things are. When this conception is held Umbra retreats.
Umbra holds sway in something like a Kantianism where that darkness outside of perception is allowed to be. That shadow that epistemologically exists in the beyond of objects, in the phantasy that perception exerts an ontological effect! This is incoherent but as has been discussed, in its base all accretions are incoherent.