CBSA and IRCC are both accountable for eligibility determination to know whether or not they're going to refer a case to the IRB to be considered for refugee determination or if the person is in need of protection. That process is done, under normal circumstances, either at a port of entry or at an inland office. If it's at a port of entry, CBSA officers will do it. If it's at an inland office, IRCC officers will do it.
The claim is made in order to trigger that eligibility interview. Then their determination is whether or not it is a matter that will have to be heard by a tribunal.