it's a widget to implement new xTML code - it opens unlimited possibilities - feature adding but also to destroy the experience.
You could for example insert new shaders, change tracking mechanisms etc etc....
Not documented for end customers for sure.
If you are missing a concrete feature, please feel free to ask.
Just to add to what Martin's comment with additional information.
This is an advanced widget, and is one where upgrade issues may result since the TML used inside some of the widgets may change.
At the moment we are still work in progress with this widget and do not have full support availability on this widget for various reasons.
For advanced users, the TML can often be found in the dist/ folder for existing widgets.It can also contain any HTML.
Note: Usage of this widget may require upgrade help in the future and possibility of running into issue during Studio upgrade.