Eventually i messaged Tiptop's tech support asking about what modules should be attached to the positive bus, what should be attached to the negative.
They just said that it doesn't matter, because all of the busses supply all of the types of current.
I've certainly had no blow outs or functionality issues with any module. I did dutifully connect the one module i have that says it wants neg to the neg bus. Why not? But then again, why? since all busses carry all polarity combinations?
As a person who likes to imagine i'm vaguely logically minded (albeit , i admit, with limited knowledge), this does leave me wondering why the different busses are defined as positive and negative polarity? Whatever the answer is to that (if there is one), it certainly isn't to do with the polarity of then current that the busses deliver, since all the busses deliver currents of all polarity.
Guess I may never know.
And it apparently makes no practical difference