The Steinitz class of a number field extension K/k is an ideal class in the
ring of integers O_k of k, which, together with the degree [K:k] of the
extension determines the O_k-module structure of O_K. We call R_t(k,G) the
classes which are Steinitz classes of a tamely ramified G-extension of k. We
will say that those classes are realizable for the group G; it is conjectured
that the set of realizable classes is always a group.