From Law wiki, the wiki for law research

A power of appointment held under a trust by a trustee; 'virtute officii' implies that it is the office of trusteeship that holds the power, not any particular person. Because the power is held by a trustee, who has a fiduciary responsibility to the beneficiaries, the trustee must at least consider the possibility of exercising that power. In contrast, a power nominatum, assigned to an individual in his private capacity, is not associated with any fiduciary obligations and need not be exercised. An exception here is a power in the nature of a trust, to which a strong expectation of exercise is attached, the result of which is possible enforcement by the courts.

See power of appointment, power nominatum, power in the nature of a trust.