LePoisson

The social security agency is the same thing here. SSN is the same as your rijksregisternummer but don't take my word for it.

https://www.ssa.gov/history/orghist.html

It's literally an identification number assigned at by the federal government to identify you - usually you get them at birth but not always. Depends on your parents I suppose.

https://www.ssa.gov/agency/pillars/support.html