I’d look at NavigationOctree.h and PrimitiveSceneInfo.h if you want to see how TOctree is used.
OctreeSemantics describe various behaviors of the Octree, maximum elements per leaf, how to compare two elements, the bounding volume for an element, etc.
FOctreeElementId is an ID that you can use to do constant time look ups into Octree to add/remove/get an object (rather than having to search for it using a bounding volume or iterator).
myOctree.GetElementById(myOctreeElementId); // Returns a reference to the object as long as that ID is valid, which you can check by calling myOctreeElementId.IsValidId()
// Do stuff
// Clean up
myOctree.RemoveElement(myOctreeElementId);