When using the FIFO resequencer, you must always specify a group XPath expression. However, you do not need to specify a sequence ID because the messages are processed according to the time of arrival to the Mediator service component that is configured for FIFO resequencing. The group XPath expression specifies where the FIFO resequencer should find the group information in the message to group the messages. No further configuration is needed for a FIFO pattern.