The Once Node is used to execute the appended part of the Flow only once during a session. It can be reset using the Reset Context Node.
The first time in a given session that a Once node is activated, the On First Time branch will be taken. All future activations of the same once node will take the Afterwards branch. It is common to use a once node to deliver a single welcome message to a new user, and ensure that the same message is not delivered again throughout the conversation.