If a `wp-cli.yml` has a _require_ in which the ABSPATH is defined wp-cli command will throw a notice.
Example:

`wp-cli.yaml`

    require:
         - abspath.php

`abspath.php`

    <?php
    if ( ! defined( 'ABSPATH' ) ) {
	    define( 'ABSPATH', dirname( __FILE__ ) . '/custom_path/wp/' );
    }

When would you want this? In short symlinks, if you include WordPress in a symlink.

WordPress tries to define the ABSPATH it uses wp-load.php and if it's in a symlink it will define the path baed on the actual location. Not the symlink path.
That will give a problem if the wp-config.php is one directory above wp-load.php.