The Language dropdown menu allows you to select the language for the entire BitTorrent interface. In order for this to work, you must have installed the language pack (utorrent.lng) into BitTorrent's settings directory, or in the same directory as the BitTorrent executable itself.
The More... button opens the download page in your web browser, where you'll find the language pack available for download.
The Associate with .torrent files button tells Windows to open .torrent files in BitTorrent. If maindoc.ico is present in BitTorrent's settings directory, it will be used as the icon for .torrent files instead of making Windows attempt to generate an icon automatically.
The Install IPv6/Teredo button makes BitTorrent install IPv6/Teredo support on the computer. Note that this option works only on operating systems released after Windows XP with at least Service Pack 2 (SP2) installed.
Check association on startup will tell BitTorrent to check associations with anything it is associated with each time it is run. If the association no longer belongs to BitTorrent at start, the user will be prompted for further action.
Check for updates automatically tells BitTorrent to connect to the BitTorrent servers to check for newer versions upon startup. Assuming BitTorrent is open for long enough, it will continue to check for updates periodically, waiting 24 hours between each update check (automatic or manual).
Update to beta versions tells BitTorrent to inform the user about beta releases when it checks for updates. Otherwise, the user is only informed of newer stable builds.
Send detailed info when checking for updates allows BitTorrent to send a randomly generated ID and the BitTorrent version and build numbers to the update server during update checks in order to count the number of people there are using BitTorrent. Whether or not this option is enabled, all communication with the BitTorrent servers are always anonymous.
The Boss-Key field allows you to select a keyboard combination that, when pressed, will toggle the visibility of all traces BitTorrent on the immediate desktop. The main window and any open dialogs, the taskbar entry, and the system tray icon all disappear when hidden, and appear again when unhidden (by pressing the keyboard combination a second time). To set this field, you must press the keyboard combination you wish to set the boss-key to. To do so, press your choice of modifier keys (any combination of Ctrl, Alt, and Shift) and the key (letters, numbers, function keys, and so on). Make sure the key combination you select is not used by another application, as that can lead to conflicts.
Append .!ut to incomplete files tells BitTorrent to append the .!ut extension to any file in the torrent contents that still hasn't finished downloading. Once a file is completed, the extension gets removed from it. Note that the state of this option takes effect immediately after the preferences are saved, regardless of whether torrent jobs are started, stopped, paused, or queued.
Pre-allocate all files tells BitTorrent to create and fully allocate every file you select to download immediately after starting the torrent job. Note that this option does not have an impact on hard drive fragmentation (advantageous or otherwise), as BitTorrent already allocates each file upon writing to disk even without this option.