Hi Mark,
The Allow Barge In property specifies if prompts can be interrupted by DTMF or not. If that property is set to true, then the message can be interrupted by a following user input request. If the property is set to false, the message will be played complete, no matter if the user presses a DTMF key or not.
For example, if you have an important notification for your users and you need that they hear it, then you set the property to false, and the message will be played entirely.
It is a good practice to set that property to false if this is the last message to play before transferring or disconnecting a call. Take a look at this blog article:
http://www.3cx.com/blog/docs/config-last-message-to-play-on-a-vad-app/
Regards,