The _ class is a part of the Logback library that can be used to include a sequential number in the log message. This can be useful if you want to track the order in which log events occurred.

Here is an example of how you might use the SequenceNumberConverter in a Logback configuration file:

<configuration>
  <conversionRule conversionWord="seqNum" converterClass="ch.qos.logback.classic.pattern.SequenceNumberConverter"/>

  <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - #%seqNum - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="CONSOLE" />
  </root>
</configuration>

This configuration file sets up a CONSOLE appender that outputs log messages in the format "%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - #%seqNum - %msg%n". The %seqNum part of the pattern is where the sequence number is inserted into the log message. Every log message will have a unique sequence number that increases for each log event.

Please note that, the output will be written to the console in this example, you can use file based appender to output the logs to a file. And you can refer to the logback documentation for more information and options available.