[D] and D what is the difference?

on the transmission there are two drive modes. one [D] and D...
how are they different?

D is Drive, in other words, 3rd gear
[D] is Overdrive, 4th gear. I think your gear selector should read

P R N [D] D 2 1

you should select overdrive, because in drive, your car will not shift into fourth, causing higher rpms, and burning more fuel. You should only use drive if you are towing, which few people(*cough* tman *cough*) try with a Buick Park Avenue.

I thought, the "Over Drive" is not actually a 4th gear but a different clutch pattern that engages in the transmission. Buicks also have the Torque Converter lockup which knocks off a few hundred RPM with or without the Over Drive.

OVerdrive is 4th gear, not some clutch pattern, the car has no clutch!

thanks guys. im getting so much help here

Well the service manual shows clutches in the automatic transmission, but I was not refering to a clutch like a manual.

looking at your post, you said "clutch pattern that engages" Sounds like a fancy way to say a different gear to me.