SceneCache ROP marks locations which appear and disappear over time.

In Houdini, SceneCache locations can appear and disappear over time when we're working with flattened hierarchies inside a SOP. Previously, this went undocumented in the file on disk, resulting in objects at such locations being visible at times that they should not. We debated off-line for a while about how to best handle this, and decided to postpone any thoughts about modifying SceneInterface to cope with dynamic hierarchy. 

This requests marks such locations with an attribute, with a bool value indicating whether or not it currently exists. While this doesn't help at all for 3d apps which can't handle dynamic hierarchy, it could be used at render time to control visibility of these locations. The attribute name is totally up for debate. I've currently called it "sceneInterface:dynamicHierarchy:exists" just to get something in there. I know David would prefer "visibility" (and to change "gaffer:visibility" to the same).