Vin: 2 S U?

Can someone tell me what these letters and numbers pertain to? When I look up parts at Mototcraft.com (and some other sites) I am often given a selection of my car and engine with three different VIN numbers/letters. It looks something like this is the selection box:

1999 Ford Taurus 3.0L Fuel Injected V6 VIN: 2
1999 Ford Taurus 3.0L Fuel Injected V6 VIN: S
1999 Ford Taurus 3.0L Fuel Injected V6 VIN: U
1999 Ford Taurus 3.4L Fuel Injected V8 VIN: N

What are those numbers? They aren't the beginning of the VIN because that's not what any Taurus VIN starts with that I've seen. Anyone know?

2 = gasoline/ethonol fuel Vulcan OHV (12 valve) 3.0L
U = gasoline Vulcan OHV (12 valve) 3.0L
S = gasoline Duratec DOHC (24 valve) 3.0L
1 = gasoline/methanol fuel Vulcan OHV (12 valve) 3.0L
N = gasoline SHO DOHC V-8 3.4L

Should be referring to the 8 digit of the VIN