want to know the detail for Exchange 2013 Folder and File structure

I found there a folder <SID>.single under each mailbox database folder in the Hard Disk Drive.

e.g. M:\MDB1\082BA04D-E4B1-45D1-EE89-3B851245B1235452.3.Single\

I would like to know what does the Exchange use it for.

Thanks.