A forecast adjustment is a judgment overlay on a rolled-up forecast total: a manager or sales user raises or lowers Best Case, Commit, or Most Likely when they expect the opportunity math to come in high or low. The opportunities themselves do not change; the adjustment sits on top of the gross rollup and does not appear in reports. Who can apply that overlay, and whose numbers they can change, depends on Forecasts Settings and the person's place in the forecast hierarchy.