The official page says
Single tickets (Einzelfahrscheine) can be used to travel once in one direction and are valid from the time they are punched in a validating machine. You may change between tram, bus and underground as often as you like, but without interrupting travel.
What does that even mean?
- What is "one direction"? This is a big city with complex transit with many modes of transportation and for example it might make sense to cut a lot of tram stops via the underground and then take the tram a few stops what I would guess could be considered backwards.
- How long is an interruption which matters here? Can I stop to buy a sandwich? What if I get lost and miss a train coming only say every 15 or even more minutes?