The residual value of a fixed asset corresponds to its resale or scrap value at the end of its economic life.
To obtain an estimate of this value, you must rely on market data, for example the resale value of a vehicle over 5 years old, or the scrap value offered by your supplier.
If it is not possible to reliably estimate the residual value of the asset (or if you wish to exclude it from the calculation): leave the value at zero.