The scene time is an internal clock that is initialized after the first Access Unit was received by client side. “0” is a value in this scene’s timeline, whereas “NOW” means “right now” at the decoding time. When the instructions are parsed on client side, “0” refers to an anchor in the timeline (i.e. the rendering of the very first Access Unit of the current scene), whereas “NOW” means “do it now” (whenever we are in the timeline when the access unit is effectively rendered).
The “0” value belongs to a synchronous process and “NOW” to an asynchronous one.

If the processing of an Access Units is slow, the anchor “0” in the timeline can be outdated when the Access Unit is effectively processed on client side. Hence the startable node would never be started.