The PDGA does not approve individual discs, it approves molds (samples of which are submitted to the Technical Standards Committee for testing). Any disc made in a PDGA approved mold is presumptively legal. Any individual disc could be tested and determined to be illegal for play (too heavy, too stiff or broken flight plate) or illegal for a particular event ( a 155 gram Magnet in a 150 Class Tournament).
As a practical matter, it is rare for a disc to be challenged or its legality ruled upon at tournaments except for what I have heard about the 150 Class events.
So once the PDGA approved the Stalker, for example, the prototypes became legal.
Thank goodness, everyone knows the first runs are always the best. :)