Sub flows help you to divide your flows into smaller and reusable chunks. When your flow grows big and complex it will also become hard to follow, debug and update. Sub flows are great to solve this problem by allowing to design your flows in a modular way.
You can add a Sub Flow node similar to other nodes from the Node palette at the left side of the Flow Designer or from the context menu by right clicking on the board.
After you add the Sub Flow node you can change its name from the node properties panel at the right side of Flow Designer.
When you double click on the "Sub Flow" node you will enter to the sub flow view. The message object from the flow will be injected to the Begin node and into the sub flow.
By default every Sub Flow has 1 output node. You can increase output ports from the properties panel.
After increasing the output ports, the sub flow view will be updated as below