Version 3.5.8
The Being Playing checkbox is a part of the Playback settings.
Definition
Controls whether the media should start playing when the shot becomes Live.
When Begin Playing is on (checked)
When this checkbox is checked, when the shot becomes live it will start playing this media.
When Begin Playing is off (un-checked)
When this checkbox is not checked, when the shot becomes live it will not start playing this media.
Default
This checkbox is on by default, meaning that media will start playing when the shot becomes live.