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The host and root hub reside on the first tier, and a connection to a hub adds another tier. The USB 2.0 specification (section 4.1.1, figure 4-1) includes the following diagram to illustrate USB topology. USB tiers result from the use of hubs, where each hub is, in fact, its own USB device as well as the start of a new layer of USB devices. Note that endpoints are not the same thing as devices.

The USB specification defines two different connectivity types in the USB tree topology: number of layers or tiers and number of endpoints or functions.

Let's take a deep dive to understand where this limitation comes from: As it happens, this is not a limitation of USBHub3+ itself, but rather a limitation of the host USB controller and the host machine's implementation of the USB controller. Some customers who have been using multiple Acroname USBHub3+ hubs have shared with us that they cannot add more than two USBHub3+ hubs to their system or that they need more USB resources.