Assign base layer - status "Blocked"

I'm playing around with my computer, and noticed that when I click "Layers>View Assignments", it shows that I removed an app layer from my PC's CVD, and that it is "done". I didn't see a Base Layer as Assigned, so I clicked Layer>Assign Base Layer and assigned our current Base Layer to my CVD. When I click View Assignments, it shows in Layer Assignments that the Type: Assign Base Layer under Status is Blocked. Can someone explain what this is? Thank you.