For questions concerning IoT applications that use microprocessors as control units.Endpoints typically use microcontrollers, and since all hubs will be based around a microprocessor, only use this tag when the question specifically relates to the processor.
For the purposes of this site, you should use microprocessors to refer to any embedded processor which runs a full Linux (or similar) operating system. Formally, this typically requires devices with an MMU (or maybe MPU). Single Board Computers (such as the Raspberry Pi), or the device in a hub or router are examples. You should use the tag microcontrollers when your question is about a small, low power processing element.