reservation of Japanese nationality
Legal procedure for children born outside Japan to retain Japanese nationality by filing a notification within three months of birth.
For a child born overseas to retain Japanese nationality, the reservation of nationality procedure must be completed within three months of birth.
国籍 means 'nationality' in general, while 国籍留保 specifically refers to the legal act of reserving Japanese nationality for a child born abroad.
Compound of 国籍 (nationality) and 留保 (reservation, retention). The term is a legal expression used in the Japanese Nationality Act.