The SKU in IoT Hub determines the number of messages a hub can handle per unit per day, and this includes both incoming as well as outgoing messages. There are four tiers. They are as follows:

• Free: This allows for 8,000 messages per unit per day and allows both incoming and outgoing messages. A maximum of 1 unit can be provisioned. This edition is suitable for gaining familiarity and testing out the capabilities of the IoT Hub service.
• Standard (S1): This allows for 400,000 messages per unit per day and allows both incoming and outgoing messages. A maximum of 200 units can be provisioned. This edition is suitable for a small number of messages.
• Standard (S2): This allows for six million messages per unit per day and allows both incoming and outgoing messages. A maximum of 200 units can be provisioned. This edition is suitable for a large number of messages.

• Standard (S3): This allows for 300 million messages per unit per day and allows both incoming and outgoing messages. A maximum of 10 units can be provisioned. This edition is suitable for a very large amount of messages:

An astute reader would have noticed that the Standard S3 tier allows for a maximum of only 10 units, compared to other standard units that allow for 200 units. This is directly related to the size of the compute resources that are provisioned to run IoT services. The size and capability of virtual machines for Standard S3 is significantly higher compared to other tiers where the size remains the same.