To OpenToonz and Tahoma2D, there are two different types of Fx's. A regular and zerary where zerary is not to have any input ports and regular does. This wasn't documented at all, but a single macro flag TOONZ_PLUGIN_CLASS_MODIFIER_GEOMETRIC is (what it appears so far) to be implementing a zerary effect that has its own level and camera. There's bound to be a ton of stuff left to discover that isn't documented at all.