Hi Wouter,
IF_RSPLS_CR_METHODS is the interface used in the implementing classes of all characteristic relationships. The *_HDB methods are uses by SAP to implement PAK support for all characteristic relationships delivered by SAP. Since this interface is also included in exit characteristic relationships you see the methods there also. These methods expose some technical stuff we don't want to expose in customer exists thus the interface IF_RSPLS_CR_EXIT_HDB was created. So follow the documentation and use the latter interface.
Regards,
Gregor