[exim] ACL Explanation

Hi all ,

Can anybody explain the VIRUS_FILESIZE_LIMIT in the following ACL

  deny  message         = This message contains a virus ($malware_name) 
and is rejected.
        log_message     = rejected VIRUS ($malware_name) from 
$sender_address to $recipients (ClamAV)
        set acl_m0      = clamd:/tmp/clamd.socket
        condition       = ${if < {$message_size}{VIRUS_FILESIZE_LIMIT}}
        demime          = *
        malware         = *

Thank you all in advance.