Vacuum resid is the heaviest of the distillation cuts. It is literallly, the bottom of the barrel.

Vacuum resid is the bottoms cut from the vacuum distillation tower.

If not upgraded, vacuum resid is blended into either residual fuel oil or asphalt. When blended into fuel oil, vac resid is valued less than the fuel oil that it produces because its high viscosity typically requires that it be blended with some higher value "cutter stock" such as kerosene. When blended into asphalt, vacuum resid's high viscosity is an advantage, so this is typically a high-value use for vac resid. However, the market for asphalt is limited, so most vacuum resid is forced to find a different home.

Alternatively, vacuum resid can be upgraded to yield light products. There are three refinery process units that can do this: the coker, the visbreaker and the resid hydrocracker.