Reference ID is a very effective way of tracking form submissions (sent and received emails) concerning a particular subject. If this feature is activated, the subject of the email you receive will contain a unique identifier, based on the time stamp, e.g.: Ref: ABC-1206090399.
ABC is the code that you type in the Reference ID box in the Settings → Notifications → Form Owner Options section. It is common for all form submissions. 1206090399 is a unique number.
Reference ID has two major advantages:
Each form submission has a unique identifier.
You can track the communication related to a certain subject. Filter your emails by the common code.