What does the octagon symbol with a thermostat and "R" represent?

The octagon symbol with a thermostat and "R" signifies a rise of rate heat detector. This specific type of heat detector is designed to monitor and respond to rapid increases in temperature, which can indicate the presence of a fire. It is typically used in areas where quick detection is critical, as it activates an alarm more quickly than other types of heat detectors when a significant temperature increase occurs.

In contrast, rapid response heat detectors focus on immediate response to both a fire's heat and smoke, while fixed heat detectors are activated only when a specific temperature threshold is reached. The fire alarm system encompasses a broader range of devices, including smoke detectors and alarm panels, not just heat detection. Hence, the rise of rate heat detector's unique capability to respond swiftly to temperature spikes is what distinguishes it within these options.