a qualified State for any calendar year shall be the amount determined by the Secretary to bear the same ratio to the aggregate unused rental reduction credits of all States for the preceding calendar year as such State's population for the calendar year bears to the population of all qualified States for the calendar year. For purposes of the preceding sentence, population shall be determined in accordance with section 146(j). (iv) Qualified State For purposes of this subparagraph, the term qualified State means, with respect to a calendar year, any State— (I) which allocated its entire State rental reduction credit ceiling for the preceding calendar year, and (II) for which a request is made (not later than May 1 of the calendar year) to receive an allocation under clause (iii). (5) Other definitions For purposes of this section— (A) Rental reduction credit agency The term rental reduction credit agency means any agency authorized by a State to carry out this subsection. Such authorization shall include the jurisdictions within the State where the agency may allocate rental reduction credit amounts. (B) Possessions treated as States The term State includes a possession of the United States. (d) Terms relating to rental reduction credit and requirements For purposes of this section— (1) Applicable percentage The term applicable percentage means, with respect to any qualified building, the percentage (not greater than 110 percent) set by the rental reduction credit agency at the time it allocates the rental reduction credit amount to the building. (2) Credit period (A) In general The term credit period means, with respect to any qualified building, the 15-year period beginning with the first month for which the qualified rental reduction agreement is in effect with respect to the building. (B) State option A rental reduction credit agency may provide a credit period for any qualified building which is less than 15 years. (3) Eligible senior unit The term eligible senior unit means, with respect to any qualified