The io part of the name is for the input and output nature of the ioStack. It has inputs for pulse counting and various types of sensors. It also has controllable outputs.

The Stack part of the name is for it ability to stack on the Push3 gateway. If you remove the lid of the Push3 you will see the same four sets of stacking pins that you see on the ioStack. If you stack the two devices they will share a power supply and pass RS-485 data up and down the stack.  You can stack many multiples of ioStacks to a single EKM Push3 gateway.