The following are key pieces in the IIoT Architecture.
A Field Device of some sort can be used to connect to a PLC. This device also needs to be able to publish Tag values to an MQTT server. Any of the following hardware can act as a Field Device:
An MQTT Broker is a server that can communicate with your Field Devices via MQTT. This can be an Ignition Gateway with the MQTT Distributor Module installed, or a third-party server, such as a Chariot MQTT Server.
A subscriber is some application that subscribes to topics in the Broker. In regards to Ignition, this is the Gateway with the the MQTT Engine Module installed.
More details on the MQTT modules can be found in the Cirrus Link documentation.