As S63 said I belive this is Sufix's (note - Sufix's) opinion on what an equivalent diameter mono would break at.
Interestingly just to make it even more confusing I see they also put a PE rating on the spool...someone will correct me but this I belive links back to a very old numbering system that was originally used for rating lines made from silk.
Well done Sufix. I dont think you could make the question of "how strong is this spool of line I am buying" more confusing if you tried !!!!!