As Flow Designer gets more real world use, the feature set continues to evolve towards the needs of production customers. In the London release, one of those features is Subflows. By its nature, Flow Designer is pleasingly fractal in nature. You build a Flow out of Actions. When you drill into Actions, the UI looks almost the same as the Action Steps build into the Action. Subflows are another layer of this fractal, allowing for reuse by building a piece of a Flow that can be reused in multiple Flows or even across the same Flow.