While having separate frontend and backend is common, sometimes it's not enough. For example, you may need additional application for, say, a blog. In order to get it:
frontend
to blog
, environments/dev/frontend
to environments/dev/blog
and environments/prod/frontend
to environments/prod/blog
.blog
instead of frontend
.common\config\bootstrap.php
add Yii::setAlias('blog', dirname(dirname(__DIR__)) . '/blog');
.