A more thorough explanation:

Definition: d'office (daw-fees). [Law French] Of office; officially. This is similar to the Latin phrase ex officio.

Examples:

• The mayor is d'office the head of the city council.
• The judge, d'office, has the power to issue subpoenas.

These examples illustrate how d'office is used to describe something that is done officially or as part of one's job or position. In the first example, the mayor is automatically the head of the city council because of their position. In the second example, the judge has the power to issue subpoenas because it is part of their job as a judge.