What is a Filepattern in Log4j 2 for?

Question

I have a question about The Filepattern for RollingFile Appender in log4j 2. The RollingFile needs an FileName and FilePattern but i dont know what the FilePattern does.I think it is just ignored. This a part of my log4j2.xml

<Property name="pattern">
    %-5p [%d{dd.MM.yyyy - HH:mm:ss}] : %m%n %ex
</Property>

<Property name="MainLoggerPath">
    C:/logs/MainLogger.log
</Property>

<Property name="FilePattern">
    C:/logs/$${date:yyyy-MM}/app-%d{MM- dd-yyyy}-%i.log.gz
</Property>


<RollingFile name="MainAppender" fileName="${MainLoggerPath}"
filePattern="${FilePattern}">

    <ThresholdFilter level="ALL" onMatch="ACCEPT" onMismatch="DENY" />

    <PatternLayout 
    pattern="${pattern}" />

    <Policies>
        <SizeBasedTriggeringPolicy size="10 MB" />
    </Policies>

    <DefaultRolloverStrategy max="10" />

</RollingFile>    

Solution

Regarding the documentation:

The pattern of the file name of the archived log file. The format of the pattern is dependent on the RolloverPolicy that is used. The DefaultRolloverPolicy will accept both a date/time pattern compatible with SimpleDateFormat and/or a %i which represents an integer counter. The pattern also supports interpolation at runtime so any of the Lookups (such as the DateLookup can be included in the pattern.