Where exactly can the session store be used?

[samuba]

I'm stumbling over Error: Function called outside component initialization a lot recently when using the $session store. And lack the understanding of where exactly this store actually can be used.

The docs state "..must be accessed during component initialisation.." and "However, it still needs to be called synchronously on component or page initialisation when the $-prefix isn't used. Use getStores to safely .subscribe asynchronously instead."

So what does "component initialisation" phase actually mean? Searching the docs for that phrase does not yield anything. I was under the impression that onMount is part of that phase but I found that the session store can only be used in the template of a component and directly inside the <script> block of a component. Everywhere else basically does not work, failing with the above mentioned error. Also using getStores does not really work because as soon as I access the store via session.subscribe() or get() it errors out with above error again.
I want to use the information from the session store in the function of a non-component file. Is there any way I can achieve that?

[mustofa-id]

session also available in page load function, if you need session information in server/endpoint, you might pass it to locals object via hooks.