This file is a per wrapper that allow to insert multiple ticket numbers in the email Subject and allow OTRS to add the email content to all the tickets referenced in the Subject. By default this won’t happen, OTRS just “deliver” the email to the first ticket found in the email Subject.
For more details please read here https://scubarda.wordpress.com/2016/07/29/configure-otrs-to-process-multiple-tickets-in-email-subject/