Realization of φ-types and Keisler’s order

We show that the analysis of Keisler’s order can be localized to the study of φ-types. Specifically, if D is a regular ultrafilter on λ such that lcf(ω,D)≥λ+ and M is a model whose theory is countable, then Mλ/D is λ+-saturated iff it realizes all φ-types of size λ.