PDP-11 Overlays

Is there any documentation as to the actual details? As I mentioned, For the RT-11 overlay handler integrated with the LINK program (both are essential), when those aspects are combined with overlay regions defined by the maximum size of the overlay in a given region, the first word of every region is devoted to the overlay (*6) number. Since that location is not available to the overlay and is unique to the overlay, the overlay handler can check to determine which overlay is resident in the region. With overlay trees in TKB, that method can't be used unless the line up the tree is intact. I am attempting to visualize the code which could determine if that is true, but at the moment it escapes me. Can anyone provide any details? Otherwise, I can see where the first overlay in any tree can be determined, but unless a given tree is enforced, there does not seem to be any way to check if overlays further up the tree are also resident? Is there a manual available on overlays using TKB that is on the internet? Jerome Fine