Custom instructions are your permanent briefing for ChatGPT. You write once who you are, what you do, and how you like answers (length, tone, format), and ChatGPT applies it to every new chat. Next to it, Base style and tone sets the overall personality.
They are broad by design. For one stream of work, use project instructions instead: inside a project they take priority over your custom instructions. You find both under Settings, Personalization.