External coupling is another type of high coupling. It exists when multiple modules share the same part of an environment that is external to the software. This could come in the form of having to use an external data format, interface, communication format, tool, or device.
Sometimes, external dependencies are imposed and unavoidable, but we should still seek to limit the number of modules that have those dependencies. Doing so will ensure that if the external dependency changes, only a limited number of modules are affected.