A custom alias is a new feature recently added to the Conquent URL shortener. If you'd prefer to use an alias that's more intuitive than the standard 'E200', such as 'myblog', you're free to input a custom alias. 'E200' will still work, but with the added bonus of having 'myblog' work as well. For example, if you want to shorten 'http://www.google.com', you could be given both 'http://t.conquent.com/google' and 'http://t.conquent.com/z100'- both would work equally well.
If an alias has been taken, the URL will be shortened to the standard 'z100' quick alias anyway, but your alias will not be set. If you'd like to try another one, simply re-shorten the same URL with a different alias. If that works, your URL will be aliased.