Note:

SoapUI (open source) is a standalone product and should not be confused with ReadyAPI Test, which is part of the ReadyAPI family.

Plugins are modules that extend functionality of tools in either ReadyAPI or SoapUI (open source). They are developed in Java and use the Plugin Framework which provides annotations and interfaces that can be used to extend or modify functionality.

They are distributed as a single JAR file containing all the plugin functionality and all the needed third-party libraries.

Plugins can be used for integration, API functionality, protocol functionality, or protocol implementations.

Integration: Integrate with other tools, platforms, or technologies.
API functionality: Tailor functionality for specific APIs.
Protocol functionality: Add new functionality to existing protocols.
Protocols: Add new protocols or API types.