Also known as: vac resid, residuum
Vacuum resid is the heaviest of the distillation cuts. It is, literally, 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 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.