Like many heuristic-based pronoun resolvers (e.g., Mitkov (1998)), they first apply a set of constraints to filter grammatically incompatible candidate antecedents and then rank the remaining ones using salience factors.
their cluster-ranking model employs only factors that capture the salience of a cluster, and can therefore be viewed as a simple model of attentional state (see Grosz and Sidner (1986)) realized by coreference clusters.