The ConversionRuleAction class is a part of the Logback configuration system and is used to define a conversion rule in the configuration file. A conversion rule allows you to define a conversion word and its corresponding converter class. Here's an example of how it might be used in a Logback configuration file:

<configuration>
    <conversionRule conversionWord="myconv" converterClass="com.example.MyConverter"/>
    <appender name="console" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %myconv{my_custom_value} %n</pattern>
        </encoder>
    </appender>
    <root level="info">
        <appender-ref ref="console"/>
    </root>
</configuration>

This example shows the use of a conversionRule element with a conversion word "myconv" and the corresponding converter class "com.example.MyConverter". Now you can use the conversion word in any part of the configuration, such as the pattern element of the encoder, this way %myconv{my_custom_value} will be converted by MyConverter class.

Here's an example of the class MyConverter:

public class MyConverter extends ClassicConverter {
    private String value;
    public void start() {
        value = getFirstOption();
    }

    @Override
    public String convert(ILoggingEvent iLoggingEvent) {
        return value;
    }
}

This class MyConverter should extend ClassicConverter and override the convert(ILoggingEvent) method to implement a specific conversion logic. In this example, the MyConverter class is storing the passed option in start method and returning the value in the convert method.

It's important to keep in mind that this is just an example of how the ConversionRuleAction class can be used to define a conversion rule in Logback configuration file, and that the actual implementation of your application may vary depending on your specific requirements.