is it one tune or many?

On a longer-than-average subway ride today I fired up Soft Machine's "Slightly All The Time" from their awesome album Third -- the tune is 18 minutes and some change and I knew I'd have time to hear it all. It's a great piece and has some unquestionably abrupt changes (partly due to the way the album was put together by splicing performances from different times + places), from the opening which seems to take its cue from modal jazz to a more Indian, psychedelic vibe and winding up in a place that must've inspired Pink Floyd (especially the Dark Side Of The Moon).

So the question arises: when do we regard a tune or piece of music as a whole and when do we regard it as a medley. Yes this might fundamentally be able to be placed under the Doesn't Matter At All rubric, but it is something to consider when composing/writing.

If for instance we took at random 3 completely different sections of unrelated pieces of music (maybe one or 2 coming from a questionable zone of "is it music?") and slapped them together as:

A B C

and left it at that do we have one piece or are there 3 pieces which occur without any pause in between? Keep in mind: we've stipulated that these are completely different and unrelated.

That's a difficult question, but I don't think the following one is. Let's take the above "piece" and structure it as:

A B C B A

Is it now one piece? I imagine that it's much easier to conceive of this as a unity, due to the structure which imposes order by some repetition.