This Data Exchange comparison help topic should provide some answers

https://jazz.net/help-dev/clm/index.jsp?topic=%2Fcom.ibm.rational.rrm.help.doc%2Ftopics%2Fr_compare_data_exchange.html

ReqIf allows the most flexibility to choose exactly which artifacts to exchange, and maintains the type system and formatting, but there are some limits like comments are not moved.

Also please note from this thread about ReqIF:  https://jazz.net/forum/questions/87001/importing-reqif-files that Tom wrote the following:

The ReqIF infocenter topic gives details on how it works:
http://pic.dhe.ibm.com/infocenter/clmhelp/v4r0m1/index.jsp?topic=%2Fcom.ibm.rational.rrm.help.doc%2Ftopics%2Fr_reqif_format.html

Notably:
Important: A ReqIF import creates types and artifacts during the first import into a project. Later imports of the same data update the existing resources, except for view column information. A ReqIF import only merges system-defined attributes and types. All other artifact types, attributes, data types, and folders are created in the project you are importing to if they have not been previously imported from a ReqIF file. For example, if you have a Priority attribute in the ReqIF file, and the current project already has a Priority attribute, the import creates another attribute that is named Priority2 .

So essentially the well-known System Types (Requirement Text, Title, Created On, Creator, etc) are able to be re-used, but at present there is no way to map user-defined types in a ReqIF file to existing types in a project. 
The recommendation is to seed any types in a project initially via ReqIF.  This associates that ReqIF object with the created type in RRC.  Thereafter, any imports using that same type (with the same ReqIF identifier) will be treated as the same.