A five board bench uses five boards; two leg boards + two apron boards + one top board = five boards.
A six board bench has a board at the bottom of the aprons so there is a storage area under the hinged top board.
My first exposure to this was a Charles Neil video on YouTube > https://www.youtube.com/watch?v=MX4fXbRNqqY
Search 5 board bench on YouTube and you will find some shorter videos. Charles Neil is kind of enjoyable to watch.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)