This assigns a new material ID (New Material ID, here: 03) to voxels with a given material ID (Old Material ID, here: 01).

With the additional option Transfer Corresponding Material, also the material of the new material ID is changed to the material of the old material ID.

Using Swap Material IDs allows to swap the old and new material ID. If additionally the option Transfer Corresponding Material is chosen, also the corresponding materials are swapped.