ChatGPT-User represents an on-demand visit triggered by a real user action inside ChatGPT, not bulk crawling. OpenAI documents it as the agent used when ChatGPT needs to retrieve a page to answer a user's request.
Together, OpenAI runs three crawlers with different jobs: GPTBot (training), OAI-SearchBot (search results), and ChatGPT-User (live user-initiated fetches). Knowing which is which prevents you from blocking the wrong one.