One crucial component of large fire response in the United States (US) is the sharing of wildland firefighting resources between regions: resources from regions experiencing low fire activity supplement resources in regions experiencing high fire activity. An important step towards improving the efficiency of resource sharing and related policies is to develop a better understanding of current assignment patterns. In this paper we examine the set of interregional wildland fire engine assignments for incidents in California and the Southwest Geographic Coordination Areas, utilising data from the Resource Ordering and Status System. We study a set of multinomial logistic models to examine seasonal and regional patterns affecting the probabilities of interregional resource assignments. This provides a quantitative and objective way to identify the factors strongly influencing interregional assignments. We found that the fire activity in the regions significantly affects response probabilities, as does the season and the national preparedness level. Because our models indicate significant unexplained variation, even when accounting for fire activity, seasonality and resource scarcity, we hypothesise that the existing system could benefit from future research.