Form element actions: Where is the documentation?

Specifically, where is the documentation for the targetName element’s ns and index variables, and the params element? I think that I’ve read everything relevant in the Adobe Developer UI Components docs, but I see no mention.

These features are used in Modifiers to alter the display of forms, like so:

'arguments' => [ 'data' => [ 'config' => [ 'labelVisible' => true, 'label' => 'Foo', 'formElement' => Container::NAME, 'componentType' => Container::NAME, 'displayAsLink' => false, 'actions' => [ [ 'targetName' => 'ns=' . $this->form . ', index=' . $foo, 'actionName' => 'openModal', ], [ 'targetName' => 'ns=' . $this->form . ', index=' . $foo, 'actionName' => 'reloadModal', 'params' => [ $params ], ], ], ], ], ],

Where is the code that reads the return values of the $this->form, $foo, and $params variables?