Would this be the same method used to assign a title to your chat based on the first prompt? It's surprisingly effective at getting the core idea most of the time.
From what I remember the title in ChatGPT gets set once after a few messages, in which case I'd assume it's generated with a special "title generation" prompt (that gets the first few messages as input).
In either case since I don't work at OpenAI I can't tell you for sure ;)