Here is an example of a class extending AppenderFactoryUsingSiftModel in Logback:

import ch.qos.logback.core.sift.AppenderFactoryUsingSift;
import ch.qos.logback.core.sift.SiftingAppender;

public class MyAppenderFactoryUsingSiftModel extends AppenderFactoryUsingSift {

    private SiftingAppender siftingAppender;

    public MyAppenderFactoryUsingSiftModel(SiftingAppender siftingAppender) {
        this.siftingAppender = siftingAppender;
    }

    @Override
    public void start() {
        // additional initialization code
        super.start();
    }

    @Override
    public SiftingAppender buildAppender(String discriminatorValue) {
        // build appender logic
    }
}

This class is an example of a custom appender factory in Logback, which is used to build appenders based on a discriminator value. It extends AppenderFactoryUsingSiftModel, which is a factory class that creates appenders for SiftingAppender. The SiftingAppender is passed in the constructor and is used to create the appenders that will handle the log events with specific discriminator value. The start method is overridden to perform additional initialization. The buildAppender method is overridden to define the specific logic for building the appenders with the specific discriminator value.

It is important to note that the SiftingAppender is used to separate logging events based on a discriminator value, and the buildAppender method is responsible for creating the appender that will handle the events for a specific discriminator value.