A Display Card controls a maximum of 16 7-segments displays. These displays have to be common ground for this card. If you need common anode, then Open Cockpits also has an USB Display Card that will do the trick.
The Display Card is connected to the Output Bus on the Master Card with a (not supplied) Display Card Connection cable (IDE cable) on which another Display Card can be connected.
A maximum of 16 displays can be connected when the dimming feature of this card is NOT used. If you want to use dimming only 15 displays can be connected because the 16th display is then used for dimming the other 15 displays.
I do not use a Display card.