What’s the difference between Firm service and Interruptible service?

Firm service is the highest quality sales or transmission service offered to customers under a filed rate schedule that anticipates no planned interruption. It is usually associated with distribution companies that serve residential customers and other "high priority end-users," but can also apply to upstream pipelines and other Interruptible service is subject to interruption at the option of the pipeline. Tariffs for interruptible service are cheaper than firm service.