Plugins are java classes contained in jar files from plugins folder that inherit IServerPlugin, ISmartClientPlugin or IClientPlugin. Since this is one file among many inside your jar, you have to indicate the which file is the plugin entry point.
The plugin jar can use Java Service Provider to expose Servoy Plugin classes. There should be a file inside jar at path: META-INF/services/com.servoy.j2db.plugins.IPlugin which contains a line for each plugin that jar should expose (a class that implements IPlugin). The plugin should also have a default constructor (with no parameters). If file com.servoy.j2db.plugins.IPlugin is missing or contains invalid entries Servoy will scan the jar for all classes that implement interface IPlugin. An example of file content (for mail plugin) is: