Something that you, personally, could be reasonably expected to know pretty much everything about, whether that's an internal system, a public API, or tool. It's up to you whether the topic is transferrable.

This is pretty like the quote from Thomas Huxley: "Try to learn something about everything and everything about something".

But I think people should choose topic carefully it should be fundamental, maybe it's soon unfortunately gone if you choose a trendy and short-lived tech.

Never heard that quote before. +1

"How small a thought it takes to fill a whole life!" – Ludwig Wittgenstein