It refers to the type of drive. A v-belt is not designed to back-bend so all v-belt drives essentially are a simple loop. Complex v-belt drives usually utilise stacked multiple belts which increase the length of the engine. A poly-vee belt can back bend so can drive a much more complex drive train where the belt 'snakes' around peripherals with only one belt.
Serpintine belt, Somtimes called a Polly V belt, Lots of small Vs rather than one big one, better than a Single V belt as it has more surface area to drive with, used much more on modern cars as it also ha less frictional losses.
Its probably called the serpintine as because of its thinner profile its easier to wrap around a pulley to get even more contact area hence more drive, so its route thru the engine is far less direct than a traditional V belt
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