A more thorough explanation:

A supernumerary witness is an additional witness who is not required by law. They are considered extra or beyond the necessary number of witnesses.

• In a court case that requires two witnesses, a third witness who provides testimony is considered a supernumerary witness.
• During a wedding ceremony, a couple may choose to have more than the required number of witnesses sign their marriage certificate. These additional witnesses are considered supernumerary witnesses.

These examples illustrate how a supernumerary witness is not necessary but can be present. In the first example, the court only requires two witnesses, but a third witness may provide additional information. In the second example, the couple may choose to have more witnesses sign their marriage certificate for personal reasons, but it is not legally required.