Generally the aim is to make it fair and equitable for everyone.
This is usually done by setting the machine on to tournament mode for those that have it. For those that don't, extra balls and so on are turned off, or if that can't be done, the extra ball is plunged but not played.
There also needs to be rules about what happens if the machine is faulty etc.
For a well-developed and well-recognised set of rules, try here
http://www.papa.org/papa10/rules.php
