Question About Incident Rules

I'm using Cloud Control 12.1.0.5.0.

I'm checking the Incident Rules of this setup and I find 2 custom Incident rules:

One for Targets Availability and one for Tablespace Usage metric.

On the Targets Availability there is a rule with action to send mail. Editing the rule I see that under the section "Create Incident or Update Incident" nothing is ticked.

On the Tablespace Usage metric there is a rule with action to send mail. Editing the rule I see that under the section "Create Incident or Update Incident" the "Create Incident" tick box is ticked.

Can someone explain what is the difference? What does create incident does? I could not find sufficient documentation.