Phillips P2 and Posidrive

I noticed they now label "Phillips" screwdriver bits as "P2" and "P2 reduced".
I noitice the head on the P2 is smaller.
WHO uses P2 "reduced" - looks like sheetrocker thing ?
I use them interchangeably.
WHY do we need 2 Phillips heads of number 2 size ?
I was in Home Depot tool corral, and they don't even have it anywhere.
I would have to go to the Woodworker Store.
Most people including myself use a Phillips screwdriver.
Seems to work but WHY was Posidrive invented ?
Better to use the correct posidrive bit - less cam out ???
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