All Out of the box Documentum object types follows the following naming convention
- All object types that are commonly used and visible to users starts with letters dm (Example dm_document, dm_sysobject, dm_user )
- All Object types that are dynamically cached (means changes to it is visible to applications and users) starts with letters dmc (Example dmc_completed_workitem dmc_jar dmc_java_library )
- All object types that read only will start with letters dmr There are only object types that starts with letters dmr they are dmr_content and dmr_containment
- All Object types that are internally used by Content server starts with the letters dmi (Example dmi_package, dmi_queue_item, dmi_session)
Ajith Prabhakar's Weblog 
As an aside, user defined types should not start with the the two characters dm… Early in my DCTM career, I was working on a demo for a company (let’s say their name was Dominion) and I created types dmn_beast, dmn_ogre, etc… A bunch of capabilities around modifying these types was locked down in DA for example just because of those 2 leading characters.