automatic gears too hav different levels (D, D3, D2, D1 (L) etc...)
D3, D2 or even D1 (L) can be used where required at extreme slope where u need to slow down.. means if u r driving in D1, then transmission will be locked in 1st gear and car will remain in Power-Gear within speed-bracket of 1st gear allowing less use of brakes...
else, in D... transmission will automatically upshift or downshift according to the acceleration level.. here u r somewhat using brakes frequently on down-hill!
ps: in some cars D3 means tht gears autoshift frm 1 to 3 up n down not locking gear only in 3rd!