  We continue the study of the rational Picard group of the moduli space of
Hitchin's spectral covers started in P. Zograf's and D. Korotkin's work [11].
In the first part of the paper we expand the ``boundary'', ``Maxwell stratum''
and ``caustic'' divisors introduced in [11] via the set of standard generators
of the rational Picard group. This generalizes the result of [11], where the
expansion of the full discriminant divisor (which is a linear combination of
the classes mentioned above) was obtained. In the second part of the paper we
derive a formula that relates two Hodge classes in the rational Picard group of
the moduli space of Hitchin's spectral covers.
