Soul Gems are used for harvesting the souls of nonsentient creatures and then binding them into the gem. This bound soul can then be used for enchanting weapons and armor, or other purposes of the arcane.
The Greater Soul Gem is the second largest Soul Gem, and can carry white souls equal or lesser than the "Greater" category. When filled with the best applicable soul, their capacity utilized in enchanting is 1200.
However, for recharging magical items, it will recharge 1200 points whichever the soul contained.