The High Level Architecture (HLA) supports the interoperation of sets of simulations within the context of a Federation Object Model (FOM), using the HLA Interface Specification services as provided by the Run-Time Infrastructure (RTI). Such simulations are federates and the set of federates is a federation. A run of a federation is a federation execution. Although the "normal" mode of operation is for a federate to operate in a single federation execution at any given point in time, the definition of HLA leaves open the possibility that a federate may be a member of multiple concurrently executing federation executions. In other words, two (or more) concurrent federation executions, of the same or different federations, could have one or more federates in common. Presumably the common federate(s) would exchange information between executions or otherwise use the events of one execution to influence another.

There are several distinct types of multi-federation executions. At the most basic level of classification, they can be broadly typed as either bridged or hierarchical. Bridged federation executions have one or more federates, called bridge federates, which are members of two (or more) federation executions. Recent literature has been primarily directed toward the common, or bridge federates which exchange (or transform) information between federation executions. In a hierarchical federation execution, one or more federates in the higher-level federation are composed of and implemented as lower-level federations, but appear as federates at the higher level. In this paper we develop a taxonomy of multiple federation executions, including examples.