It's very simple.
You can create any additional method to the policy.
For example if you have some categories to which a user can subscribe, you can add a subscribe function.
public function subscribe(User $user, Category $category)
{
return $category->isPublic();
}
And you can use this function to check authorization in your controller.
public function subscribeToCategory(Request $request, Category $category)
{
$this->authorize('subscribe', $category);
// your code if the authenticated user is authorized
}