When writing some flows for Node-RED controlling my Philips Hue bulbs; I wanted to read from a bulb do some work then write that same colour back to the bulb. When you perform a GET on a bulb, you get all of the information, whilst good this means it is a bit more inconvenient to PUT that back into a bulb as that API has a slightly different resource shape where the colour mode is determined by which values are submitted.
After using a group of nodes in one flow which did exactly this, I needed the same block in another flow. So I created a sub flow by selected those nodes, clicking the hamburger menu in the top right and choosing 'Subflows > Selection to Subflow'. This replaces the selected nodes with a single node which you can edit in a 'subflows' menu group in the left pane.