I haven't done the JLPT for a long time but I'm wondering if it's one of those boxes you get on application forms that is for the purposes of the administrators rather than something you fill in - you're not going to yet know your examination registration number if you're applying for the exam. The Japanese at the bottom says year, area, country, region, city, location, level, individual number, each of which I imagine is assigned a code by the administrators. There also isn't a translation into English, which suggests you're not to fill it in.
Anyone here done the exam recently?