As used is this agreement:
(a) "State" shall mean a state of the United States; the United States of America; a territory or possession of the United States; the District of Columbia; the Commonwealth of Puerto Rico.
(b) "Sending state" shall mean a state in which a prisoner is incarcerated at the time that he initiates a request for final disposition pursuant to Article III hereof or at the time that a request for custody or availability is initiated pursuant to Article IV hereof.