Only once — an alert is triggered only once. Alert conditions have to match the exact parameters that you set for it to be triggered.
Once per bar— the system checks every bar and triggers an alert whenever conditions are met (no more than once per bar).
Once per bar close — same as above, but the bar needs to close for an alert to be triggered.
Once per minute or every time— the system checks conditions every minute and triggers an alert whenever conditions are met.