Not really assigned at birth, but at the time you first request the card. It's a very very very crappy database ID number: -There's a significant amount of dup

Not really assigned at birth, but at the time you first request the card. It's a very very very crappy database ID number: -There's a significant amount of duplicates, enough that if your database is big enough you will find one sooner or later. -Format is variable: There are older 7 digit numbers, modern 8 digit ones, some people pad the 7-digit with a 0, some don't, some consider the CRC letter part of the key, some don't, some just append the letter, some hyphen it. If you deal with manual data entry anywhere, you will suffer. -Foreign people exist: You start using their passport number, the format of your key is now completely arbitrary, can't validate it. If you were using 8 digit NIFs as keys now some countries use 8 digit passports, increasing your chances of duplicates. -Foreign people stay: They get a resident card and they use that. Format is almost like NIF but not really so you need to account for that. Someone you registered initially with a passport now has the card and registers for something else with it. Have fun cleaning their duplicated identity from the whole system. -Foreign people become Spanish: Now they get a NIF. If you have dealt with them for a long enough time, have fun again fixing their records for the second time.