[eluser]vs.dev[/eluser]
When you define in the environment this also defines which config file will be used:
- default application/config/config.php
- development application/config/development/config.php (if it exists)
Error logging depends on these config variables in config.php
$config['log_threshold']
$config['log_path']
$config['log_date_format']
So for different environments (defined in index.php) you can set different error reporting options (always index.php) and different logging options (which are defined in config.php)