The SubDialogueTree node is to be used when your Dialogue Tree is an asset and you want the agent of the Behaviour Tree to be the instigator of that dialogue and the agent already is a Dialogue Actor (has the component), and you also want to parametrize some of the variables of the Dialogue Tree through the variables of the Behaviour Tree (Variables Mapping in the inspector of the SubDialogueTree node).
The Start Dialogue Action on the other hand is for executing a Dialogue Tree Controller separately (either in the scene, or a prefab) like you did.
Gavalakis
