I'd guess that warrior and religious types would cover everything. Maybe like acolytes and privates build things.
I think it came about when JMS had the idea to make the Minbari ''based on three''. It made no sense for the government to not be ''of threes''. So he had to add the workers quick.
It's kinda like the odd Star Trek thing where ''all'' the Klingons are ''warrior born'', but then who does all the work in society?