It would override your robots.txt file, though it won't delete it, it would just be ignored if it's there (provided you do the mod_rewrite)
This is an example of what it generates
Code:
User-agent: Mediapartners-Google
Disallow:
User-agent: *
Disallow: /account*
Disallow: /help*
Disallow: /misc/style*
Disallow: /login*
Disallow: /logout*
Disallow: /lost-password*
Disallow: /register*
Disallow: /reports*
Disallow: /search*
Disallow: /conversations*
Disallow: /css.php
Disallow: /cron.php
Disallow: /admin.php
Disallow: /js
Disallow: /styles
Disallow: /attachments/*
Disallow: /online/*
Disallow: /recent-activity/*
Sitemap: http://yourdomain.com/sitemap/sitemap.xml.gz
There is a textarea to add custom rules should you need to disallow any url.
By default it does not block member pages and attachments, but it has the options to do so. Files like css.php, styles and help are always blocked.