Any other Codebeamer fields which are not mapped to a specific MS-Project field are exported as custom fields. Such fields get a "cb_" prefix, for example "Submitted by" appeas as "cb_Submitted by" in MS-Project. Such fields can be displayed and used in various views of MS-Project:

These are available for convenience and for analysis, yet these are not used nor imported back later from MS-Project during roundtrip.