For questions concerning IoT applications that use microcontrollers as control units. Almost every IoT device has an embedded microcontroller. Thus, use this tag when the usage, development or constraints of the microcontroller are at the core of the question.
For the purposes of this site, you should use microcontrollers to refer to any embedded processor which does not run a full linux (or similar) operating system. Formally, this typically excludes devices without an MMU (or maybe MPU). For generic questions about processors in Single Board Computers (such as the Raspberry Pi), use microprocessors. A microprocessor is likely to be used in a hub or router.