TYPO3 Core

That would be the purpose of a Dataprovider.
See https://docs.typo3.org/typo3cms/CoreApiReference/ApiOverview/FormEngine/DataCompiling/Index.html

Does this help?

Thanks for that. You're probably right. I had a glance at the documentation and tried to wrap my head around this. It seems like a very powerful mechanism, but I must admit that I'm fairly new to the concepts described there. I still struggle a bit with how you'd find out, where exactly to locate your own provider and what data formats are required etc. But I didn't have time yet to really look into that.

So we can close this issue?

Yes

Why was this issue closed? There is no solution, except Mathias' link. If that is the way to go, it should go into the documentation to this place:
https://docs.typo3.org/typo3cms/TCAReference/ColumnsConfig/Type/Select.html#multiselectfilteritems
with a good example.
Please re-open this issue - thank you!

Even though I agreed to close the issue, I would definitely support Robert's request.

I think without prior knowledge about the FormEngine and its data compilation process, no average developer will know about this option just from consulting the TCA reference.
And a good example is always helpful. :-)

I created a github issue for documenting this in the TCA reference. If you want to get notified if progress is made in this area (e.g. Pull Request submitted), you can Subscribe the github issue.