  We study subsets of Grassmann varieties G(l,m) over a field F, such that
these subsets are unions of Schubert cycles, with respect to a fixed flag. We
study the linear spans of, and in case of positive characteristic, the number
of points on such unions over finite fields. Moreover we study a geometric
duality of such unions, and give a combinatorial interpretation of this
duality. We discuss the maximum number of points over a finite field for the
Schubert unions of a given spanning dimension, and we give some applications to
coding theory. We define Schubert union codes, and study the parameters and
support weights of these codes and of the well-known Grassmann codes.
