A Closed Shop or not?

Passing this closed shop in the village today it took me back to a question I had in an exam many years ago in history. "What is a Closed Shop".
I remember making up some story about economic instability,trade going to supermarkets ,the consumer looking for something different and a few other long winded statements.
Mark given was zero.
Answer should have been,an agreement where only union members will be employed in that establishment
Well,I'm more of a technical type anyway!