Transfer Codes: Transfer codes are automatically assigned whenever a forwarding phone number, voice mailbox, and desktop user are created on your Talkroute account.  These codes can not be edited or changed.  If you blind transfer a call to a forwarding phone number on the Talkroute account, and that agent does not pick up the call, that will go to that agent's personal voice mailbox. 
For more information on Live Call Transfer, please see this tutorial: How to Transfer Live Calls.

Multiple Digit Extensions: Multiple digit extensions can be three or four digits long.  You can choose the number combination.   If you blind transfer a call to the multiple digit extension, and no one answers this call, you can choose a Talkroute voice mailbox in that extension to send a call to.
For more information on Multiple Digit Extensions, please see this tutorial: How to Add Multiple Digit Extensions. 

Please note: Transfer codes will be disabled for your forwarding phone numbers, voice mailboxes, and users if you add more than 99 to your account.  You may still transfer live calls to forwarding numbers with disabled transfer codes, but you will first need to associate each number to an extension and utilize the extension number to make your live transfer.

Learn more about Blind Transfer here: Blind Transfer Behaviors