Hi Pukie and yes it has been covered many times in the past.
In short WGT uses the "One Ball Rule" as is used in all PGA events and Major Tournaments, as well as many local competitions. As such if you lose your last ball and without the exact same ball and model you are disqualified. WGT allow you to finish but with only the starters, better than having to quit.
In addition the courses are set up as per for the US Open, again simulating the major.
You are also given a warning when it is your last ball, so ignore at your peril.
Hope this explains it but use the search for more info