score:8
Do I understand correctly, that my Aufenthaltstitel will be sufficent, or do I also need a passport?
No, that's not the meaning of this blurb. They simply mean a national ID card as issued by almost all EU countries. A residence permit can in some limited cases play a similar role and I have even seen someone talk a bus driver into letting them onboard with a scan of a passport on their phone but you shouldn't expect transporters to accept any residence permit for that purpose. That's not what this means.
Upvote:2
From a practical point of view, you won't need any documents as nobody checks anything. I've travelled extensively between countries in the Schengen zone and not once have I had to produce my passport at a border when coming by train or car (one exception during a lockdown and me travelling on an emergency). Even on plane travel it is rare.
However, the law in several countries requires non-citizens to have a passport with them at all times. In Austria, if you are a EU citizen, you have to have a passport with you. If you are a non-EU citizen, you have to have your Aufenthaltskarte with you. (see here or here).
So even if you don't cross the border, you need to have your card on your person. I don't know the laws in Italy or Slovenia. Just make it a habit to take your card, because why not?
(of course, your bus company may demand that you have a passport, or a red hat and a teddy bear - as long as they're not a public transport and clearly post them, they can make whatever rules they want)
Update: According to Mark's comment below, the information about having to carry an ID document is apparently wrong for Austria, despite multiple online sources saying so - see here for an official government page. You might still want to check what the law in Italy and Slovenia says.
Update2: An important comment was moved to chat: In Slovenia, you apparently do need to have an ID card.
Upvote:3
The rules are constantly changing depending on the nationality of your passport and the current COVID restrictions. Best practice, if traveling to another country take your passport.
For up to date information on the current Schengen-area requirements and general EU passport/freedom of movement, see here: https://home-affairs.ec.europa.eu/policies/schengen-borders-and-visa/schengen-area_en
You can see the currently accepted list of identification at the portal here, per EU country. https://www.consilium.europa.eu/prado/EN/search-by-document-country.html
In general, travel between Schengen-area countries doesn't require document checks. Each country has their own documentation requirements. The EU has a 'freedom of movement' for nationals but non-local residents (meaning foreign nationals with an EU-member state issued residency card who are visiting) have different per-country restrictions on the documents required while visiting from places like hotels and airlines, especially in non-Schengen countries.