  To every abelian subvariety of a principally polarized abelian variety $(A,
\mathcal{L})$ we canonically associate a numerical class in the N\'eron-Severi
group of $A$. We prove that these classes are characterized by their
intersection numbers with $ \mathcal{L}$; moreover, the cycle class induced by
an abelian subvariety in the Chow ring of $A$ modulo algebraic equivalence can
be described in terms of its numerical divisor class. Over the field of complex
numbers, this correspondence gives way to an explicit description of the
(coarse) moduli space that parametrizes non-simple principally polarized
abelian varieties with a fixed numerical class.
