HLA - High Level Architecture
The HLA (High Level Architecture) is a more recent standard for interoperability among simulations. Rather than a networking protocol (wire standard) like DIS, HLA defines an architecture with a set of API (Application Programmer's Interface) Standards. Simulation applications (known as federates in HLA) communicate by making calls to the HLA APIs. A piece of software known as the RTI (Run-time Infrastructure) implements the HLA API, and is responsible for transporting data from one federate to another. Like DIS, the HLA Standards are owned by IEEE. There are three documents that comprise the HLA Standard, all available from IEEE.
IEEE 1516-2010 - IEEE Standard for Modeling and Simulation (M&S) High Level Architecture (HLA) - Framework and Rules provides the rules and definitions for implementing and using HLA.
IEEE 1516.1-2010 - IEEE Standard for Modeling and Simulation (M&S) High Level Architecture (HLA) - Federate Interface Specification defines the various services provided by an HLA RTI, and contains the APIs.
IEEE 1516.2-2010 - IEEE Standard for Modeling and Simulation (M&S) High Level Architecture (HLA) - Object Model Template (OMT) Specification defines the format used for describing object models in HLA. An object model dictates what kinds of data a particular set of HLA federates will be exchanging.