By default the base unit supports up to 2 additional wired sensors. If you want to add more than 2 sensors to a base unit, then the modules enabled you to extend it to 8 sensors (sensor hub) or 30 wireless sensors (wireless hub).
One expansion module per Sensorgateway (base unit) is supported
By default the SensorGateway can host up to 2 external sensor probes. In some cases 2 sensor probes may be insufficent. This is where the Sensorhub comes in. Should not be used in environments running Wifi on 2.4Ghz due to possible interference.
Originally designed for monitoring remote distributed network hubs for the IDA Smart Nation project in Singapore, the Multi-Sensor & Hub is the result of a long analysis of what a sensor box should be able to monitor inside a server room or remote cabinet.