For example, on "Drum"
We are buying oil as a drum, but we are selling it as per liter. Also, this drum can vary per vendor.
E.g.:
Vendor 1 sells 1 drum = 500 liters
Vendor 2 sells 1 drum = 1000 liters
Also in "Roll" we are buying it in a roll and selling it as per meter. Same scenario with drum.