The Event Registration Form part includes an Acknowledgement Email that can be sent to the event registrant upon successful registration.
When editing this email, there is a "More Options..." button. Clicking this button reveals the following two choices:
Send this acknowledgement email only when a summary email is not available
Always send this acknowledgement email
Choosing the second option (send always) and clicking "Save" may appear to save your selection, but logging out and logging back in to NetCommunity and editing the email again may show that the selection reverts to the first option.
Note the help text that shows above these two choices:
"Select when to send this acknowledgement email. This applies if this part uses a Payment part that is configured to send a summary acknowledgement email."
If your Event Registration Form is not configured to use a Payment Part to process payments (i.e. it's set to "Proceed directly to payment"), the choices under the "More Options..." button in the Acknowledgement Email do not apply and will not save.
In this case, the Acknowledgement Email associated with your Event Registration Form part will always be sent.