“The koseki is Japan’s family registration system. All legally significant transitions in a person’s life — births, deaths, marriages, divorces, adoptions, even changes of gender — are supposed to be registered in a koseki; in fact, registration is what gives them legal effect. An extract of a person’s koseki serves as the Official Document that confirms to the Rest of the World basic details about their identity and status.
Need to prove when you were born? Koseki extract. Need to show you have parental authority to apply for a child’s passport? Koseki extract. Want to commit bigamy? Good luck; the authorities will refuse to register a second marriage if your registry shows you are still encumbered with a first.
Compared to “event-based” Official Documents (birth certificates, divorce decrees and so forth) that prevail in places like America, the koseki is more accurate. An American can use a marriage certificate to show he got married on a particular date in the past but would struggle to prove he is still married today. A koseki extract, on the other hand, can do just that.”
Source: Japan’s koseki system: dull, uncaring but terribly efficient | The Japan Times