Assuming the show is a worthwhile property, is well put together, and enough people come to see it, each week the Net Box Office Takings, less the Weekly Running Costs, will provide a Profit. As a unit holder you will receive 1/700 of this Profit until Recoupment - the point at which your investment has been paid back in full.
After Recoupment the Producer takes 40% of this Profit for his insight in finding the material and expertise in guiding its production; and you continue to receive 1/700 of the remaining 60% of the Profit for as long as the run continues - in the case of The Phantom Of The Opera probably for ever.
If on the other hand the show is a disaster and folds, then not only are the remaining physical assets worth nothing, but the producer has to pay people to remove them from the theatre and throw them away. So you get nothing.