This paper describes an ongoing effort to define an interoperability specification for the Marine Corps, specifically the Marine Air Ground Task Force (MAGTF) training community. The focus of the research effort is on combined arms training in a distributed Modeling and Simulation (M&S) environment. The interoperability specification is built upon the High Level Architecture (HLA), and it includes the development of a Conceptual Model of the Mission Space, an HLA Federation Object Model (FOM), and an interoperability agreements specification to compliment the FOM. The development of this specification is addressing a wide range of considerations. It is addressing Marine Corps operations, doctrine and programmatic considerations, which emphasize the need to address a heterogenous set of mission domains in order to support such concepts as asynchronous warfare and operations other than war (OOTW). It is addressing issues of fidelity related to modeling the warfighter's perspective of the mission, support of an integrated learning methodology to facilitate control of the environment and measurement of performance and effectiveness, and support of scalable training at both the team and individual levels. This project is also addressing issues related to compatibility with training systems that are currently in the development cycle, as well as implementation of a flexible and evolutionary design to accommodate technological advancements. The primary purpose of this specification is to facilitate standardized interoperability of future Marine Corps training systems. This paper is a follow-up to one that was published last year entitled "Baseline Interoperability for Marine Corps Air and Ground Simulators: The Marine Air Ground Task Force Federation Object Model (MAGTF FOM)".