Sustainer Giving Patterns vs. Duration Period?

We recently had a donor create a sustaining gift with a monthly duration of 2. We hide that option from the front end with javascript. He must have had JS disabled in his browser.

That's the context.

After looking at documentation and definitions: What is the difference between Sustainer Giving Patterns and Duration Period in the Flexible Sustainer Donation Level form element?

I mean, I see the difference in the form elements, but are they not somewhat redundant? At the very least, shouldn't Ongoing in Duration Period supersede whatever integers are entered in the Patterns?