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.