A unique identifier for rhapsody model elements

If you create a new model element, and then look in the Rhapsody file that contains it. You'll notice two IDs.
One is a Rhapsody generated GUID. Ex: "GUID 91cfe3f9-534e-4ab2-8426-819865ffdc6e". The Rhapsody client will assign a GUID to any newly created model element. It will not change (even if the model element moves around in the model).
(See this on how to force change a GUID if 'copying' models however: link)

When a model project is RMM enabled, it also gets assigned a RMM server ID. This ID is also constant and is expected to map to a GUID (and remains the same even if the model element gets moved around in the model itself).

This RMM server ID is much more human readable however (similar to work item IDs). Ex: 1015876.
This ID is also shown in the URL when viewing the model element in the RMM web-ui for example.

Example URL:
https://HOST:9443/ccm/web/projects/ProjectA#action=com.ibm.team.rmm.designs.viewResource&resourceUri=https%3A%2F%2FHOST%3A9443%2Fccm%2Fresource-rmm%2F1015876&oslc_config.context=https%3A%2F%2FHOST%3A9443%2Fccm%2Frtcoslc%2Fscm%2Fconfig%2Fs%2F_bV7-MYsrEe2CQcNgz1Kzcw

Note: There is some rare exception where model elements created in the past (ex: RMM 6.x timeframe) may have the same RMM ID (i.e. two different GUIDs with the same RMM ID). This is a rare scenario, and often prevents change set delivery. To fix this, a user would have to run Java API (RhapsodyModelManagerDataValidation) on the Rhapsody client to repair conflicting RMM IDs. This will update the files on disk with the new IDs which can then be checked-in and delivered. (instructions here: link)