Technically speaking, it's all data coming from the mobile device. Hotspotting occurs on my device without their involvement since it's a feature of the hardwar

Technically speaking, it's all data coming from the mobile device. Hotspotting occurs on my device without their involvement since it's a feature of the hardware, not the cellular network. In effect, T-Mobile sells you a device and then rents out the capability of your own device. One right way to do it is to put a hard cap on data usage but then they couldn't use the term "unlimited" and T-Mobile would lose customers. The other right way to do is soft-cap all data coming from the device, which would allow "unlimited".