[math-fun] q-analogue of MacMahon's plane partition formula

.. as mentioned at the end of Diagonal Sums of Boxed Plane Partitions www.combinatorics.org/Volume_8/PDF/v8i1n1.pdf refering to M.D. Blum , unpublished notes (1996) could anyone explain how to interpret the 'sub_q' in (i+j+k-1)_sub_q. I would like to express the number of plane partitions ( of n ) that *just* fit inside a k by k by k box, not the total count irrespective of n as in McMahon's formula. Wouter.