Always On is the term used to describe a Workflow that executes continuously until explicitly stopped. Workflows that are enabled for Always On usually commence with a Trigger Node. When a sequence of execution completes, control is returned to the Trigger Node which blocks until the trigger condition is met.
To permit a Workflow to run in Always On mode, toggle the Can Run Always checkbox in the Workflow Design Settings Pane.
Once this setting is enabled, users will be able to start a Workflow in Always On mode by clicking the Turn On button on the Workflow Landing Pane. The Workflow will begin to execute immediately and will automatically be restarted within 15 minutes if it is restarted due to being idle, platform maintenance or an error.
By default, the Always On Workflow will execute under the Production Site Environment. If you'd like to have the Workflow run under a different Site Environment, choose the appropriate one in the On Demand and Always On Site Environment Dropdown in the Workflow Design Settings Pane.
Workflows that support both Run Now and Always On should have both the Start and Run Now Execution Flow Outputs connected on the Start Node.
See https://flowgear.me/s/7K7hGzf for an example of this pattern.