R
radiohead
There isn't, you are correct. But the term is generally used and a majority of electricians know that when a '16th board' is referred to it is describing a split insulated DB with usually only socket outlet circuits RCD protected. A 17th board usually means an insulated dual RCD board, and an 18th means an all steel enclosure.It is highly unprofessional in my eyes to use the terms 16th, 17th or 18th Ed boards as their is no such thing.
Boards are referred to in this way because those DB configurations were/are the most commonly used. No, not correct terminology, but highly unprofessional is a bit OTT.