[arthurcolle]

The assistant has its own "instructions" (replacement for system prompt)

and then on each run, you have the option to add more guidance to the run explicitly, without modifying the assistant instructions (system prompt)

It's a little bit different but kind of the same

[baobabKoodaa]

No, adding run instructions will replace existing instructions for that run

[arthurcolle]

That's what they say ;)