This paper introduces two generic mechanisms allowing to compare security policies from a semantical point of view. First, a notion of embedding is defined in order to compare policies over a common domain. Then, interpretations of security policies are introduced in order to consider their properties over arbitrary domains. Thus, combining interpretations and embeddings allows to compare policies expressed over different domains. Along the lines of this paper, we illustrate our definitions by defining a flow-based interpretation of access control and by comparing classical access control policies according to a hierarchy of abstract flow policies, thus characterizing flow properties which can be ensured by access control policies.