The Pub/Sub paradigm is a part of MAS Messaging, and is implemented using the MQTT protocol. Why MQTT? It is lightweight and simple for constrained communication environments such as IoT. Not to mention it has a small code footprint where network bandwidth is at a premium.
If you watch your app data move through MAS Messaging, it would look something like the diagram below. Where communications and security are handled on the CA Mobile API Gateway.
All of the backend complexity is abstracted away, so your interaction with the iOS Mobile SDK looks simply like this: