One pole per deck is a simple concept if you look at a rotary switch wiper.
Take the rotary switch below you purchased. Each "deck" is a separate layer, and in your switch would have 12 contacts around it and one common wiper to connect to them as you turn the switch. One of those contacts around the switch will be the common that connects as you turn the switch, the other twelve will be the individual positions. There are three decks, so there are three rows of contacts.
