Section 10-3.4. Classification of powers of appointment as to duty to exercise;

imperative and discretionary
    (a) A power of appointment is either imperative or discretionary.
    (b) A power of appointment is imperative if the instrument creating it
  imposes  on  the  donee  a duty to exercise it, and it may be imperative
  even though it is exclusive.
    (c) A power of appointment is discretionary if the donee is authorized
  to exercise or not to exercise it.