The built in plugins/behaviors are those that comes with C2. (The mouse, sprite, text, textbox objects, etc...)
If you haven't downloaded any third-party plugin from the plugins forum, then all your objects are allowed except the four we already mentionned.
You can find the full list of built-in plugins in the manual's plugin reference part. Any other plugin is a custom/third-party plugin.
I'm not saying such a list wouldn't be implemented. I'm just pointing out how to workaround in the meantime, until this feature gets added, if judged adequate by Tom when he'll have the time to implement it.
Tom's todo-list is as long as Ashley's.