dm is an acronym for the D3 data manager account which contains the files necessary for basic system operation.
D3 makes extensive use of its own data management capabilities to manage system-related data in the dm account.
There are three files on the dm account that are used by the system and the users. These files are tcl-stack, users, and pibs and should be copied using the t-dump command prior to performing an operating system upgrade. After a full restore, these files should be loaded using the t-load command.