Hi,
It holds the current search instance settings. I would avoid using that if possible, as the search settings array keys often change when new options are incorporated and older ones get removed – therefore it is not documented.
The full list of keys and default values you can find in the wp-content/plugins/ajax-search-pro/backend/settings/default_options.php file, from line 207 to 991.
If you are looking for a specific setting, let me know, I might be able to help.