NO stands for ‘No Overflow’. This LED lights up as long as there is no stack overflow. When a number is pushed out of the top T‑register your answer might be wrong due to stack overflow, and this is indicated by an extinguished No Overflow LED.
When the NO LED is off the calculator functions normally. Before starting a longer calculation press the CLR‑key: as long as the red LED remains on you can be sure that your answer is not flawed by stack overflow.
NO: No Overflow !