Answer:
Page 149 in the 2023 GIR
If there are multiple applicants (i.e., an applicant and one or more co-applicants), the Financial Institution reports the ethnicity, race, and sex information for the applicant and the first co-applicant listed on the collection or Application form. If an applicant did not provide the information for an absent co-applicant, the Financial Institution reports that the information was not provided by applicant in mail, internet, or telephone Application for the absent co-applicant. If there is only one applicant, a Financial Institution reports that there is no co-applicant. Appendix B to Part 1003.