Message Action Entities are sub-type records that define the service where the action will be available. See the About the Message Actions Form for more details.
Message Action Entities Form 
General Tab
Entity ID (
...
Required)
Defines the service where the message action can be execute from.
Auto Add
When selected the message action is automatically added to the Actions tab of any Aptify Message Form or Message Temeplate record for the specified entity.