framework/base/Module.php
public function get($id, $throwException = true)
    {
        if (!isset($this->module)) {
            return parent::get($id, $throwException);
        }

        $component = parent::get($id, false);
        if ($component === null) {
            $component = $this->module->get($id, $throwException);
        }
        return $component;
    }
public function get($id, $throwException = true)
    {
        if (!isset($this->module)) {
            return parent::get($id, $throwException);
        }

        $component = parent::get($id, false);
        if ($component === null) {
            $component = $this->module->get($id, $throwException);
        }
        return $component;
    }
{@inheritdoc}

Since version 2.0.13, if a component isn't defined in the module, it will be looked up in the parent module.
The parent module may be the application.