You are a nurse, and you begin almost every chat by explaining your job and asking for short, jargon-free answers. You want to stop retyping it. What should you set up?

The principle — anything you would type at the start of every conversation is configuration, and configuration belongs in settings rather than in your fingers.

Why the key is correct — OpenAI describes custom instructions as a place to share anything you would like ChatGPT to consider in its response, and states that they are applied immediately to all chats. Your role and your preferred answer format are exactly that: stable facts about you that should shape every reply without being restated.

Why the others are wrong — a project would scope the preference to chats started inside it, which fails the moment you ask something quickly from your phone. A pasted template does not remove the repetition, it just makes it faster, and it still depends on you remembering to do it. Memory is genuinely useful, but it works by picking things up from conversation, which makes it strong on incidental detail and weaker as a guarantee about how every answer is formatted. An explicit standing rule is what the instructions field exists for.

Remember this — the test is whether you would say it to every new conversation. If yes, it is a custom instruction. If it is only true of one body of work, it is a project instruction instead.

Sources — OpenAI's ChatGPT Custom Instructions article.