  The main result of this paper is a new parameterization of the orbits of a
symmetric subgroup on a partial flag variety. The parameterization is in terms
of certain Spaltenstein varieties, on one hand, and certain nilpotent orbits,
on the other. One of the motivations is related to enumerating special
unipotent representations of real reductive groups. Another motivation is
understanding (a portion of) the closure order on the set of nilpotent
coadjoint orbits.
