IEC 61499-4 describes the rules that a system, device or software tool must follow to be compliant to IEC 61499. These rules are related to "interoperability, portability" and "configuration". Two devices are "interoperable" if they can work together to provide the functionality specified by a system configuration. Applications compliant to IEC 61499 have to be "portable", which means that they can be exchanged between software tools of different vendors considering the requirements for software tools described within IEC 61499-2. Devices of any vendor have to be "configurable" by any IEC 61499 compliant software tool.