Defining dynamic models for traits

I am learning about traits and need some help to understand defining models for re-usability.

Both of the following examples to define the model seem to work but I don't really understand the differences. Is there any reason why I should use one methods to over the other?

// define the model and namespace
private $modelA = '\App\Models\Category';

public function getItemsA()  {
    return $modelA::all();
}


// define the model class
private static $modelB = Category::class;

public function getItemsB() {
    return self::$modelB::all();
}