I've meet hundreds of Fnus in my life. I used to work for Immigration and Customs and went to countless fishing vessels and cruise ships... The problem is that countries like India or Indonesia have different naming conventions that allow for people to carry a single name. In Indonesia a name like Sumarto can convey way more information that perhaps a common Firstname+Middlename+Lastname name.
Many software implementations are build to work with traditional patronymic names and will act erratically when presented with a passport with no lastname. Then the fix is to either use Fnu or another symbol like an asterisk. I have seen software freaking out when parsing the MRZ of indian passports where, like in this case, the person have all names on a single data line, making it appear like this person has got no first name.
On cruise ships you get giant manifests with dozens of Fnus at the time...
