  The two best studied toric degenerations of the flag variety are those given
by the Gelfand--Tsetlin and FFLV polytopes. Each of them degenerates further
into a particular monomial variety which raises the problem of describing the
degenerations intermediate between the toric and the monomial ones. Using a
theorem of Zhu one may show that every such degeneration is semitoric with
irreducible components given by a regular subdivision of the corresponding
polytope. This leads one to study the parts that appear in such subdivisions as
well as the associated toric varieties. It turns out that these parts lie in a
certain new family of poset polytopes which we term relative poset polytopes:
each is given by a poset and a weakening of its order relation. In this paper
we give an in depth study of (both common and marked) relative poset polytopes
and their toric varieties in the generality of an arbitrary poset. We then
apply these results to degenerations of flag varieties. We also show that our
family of polytopes generalizes the family studied in a series of papers by
Fang, Fourier, Litza and Pegel while sharing their key combinatorial properties
such as pairwise Ehrhart-equivalence and Minkowski-additivity.
