Inspired by this question about UK inter-city rail.
In Berlin, a single-trip ticket is valid for: "a single journey towards your destination", changing trains/buses "as many times as required", for 120 minutes (stopping in the middle and then getting back on is explicitly permitted, provided it's within this time window). The ticket records your origin, but not your destination. Tickets are checked "randomly" by plainclothes inspectors.
How can I tell which routes I am permitted to take?
Here is a U-Bahn/S-Bahn map for reference. I am not permitted to reproduce the map here.
Suppose that I am at Wedding (top-centre of the middle zone) and I want to go to Tempelhof (bottom-centre of the middle zone). I can take the U6 line (purple) which cuts through the middle of the map. No changes are required.
Alternatively, I can take the Ringbahn (brown lines) in either direction. Both directions look about the same length. No changes are required. Are these routes also allowed?
Suppose that I am at Wedding again, and I want to go to Innsbrucker Platz (3 stops left of Tempelhof). The smallest number of changes required is 0 (taking the ring). If I take the U6 to Tempelhof and then the Ringbahn to Innsbrucker Platz, I might get to Innsbrucker Platz sooner, but this route includes a change of trains, which is not required. Is this route allowed?
If "unnecessary" changes of trains are allowed, is there a limit? Can I get from Wedding to Innsbrucker Platz via Wedding -> Stadtmitte -> Potsdamer Platz -> Yorckstr -> Bayerischer Platz -> Innsbrucker Platz? (4 changes, when 0 would do)