The useful answer: RST is the traditional readability, strength, and tone report: voice contacts normally use RS, while Morse contacts add the tone digit.
What changes in real operation
The report gives the other operator a compact description of how the signal is being received at that moment; it is not a calibrated field-strength measurement.
The shortcut that causes trouble
Automatic “five-nine” reports are common in contests, but outside that context they can hide useful information about readability, noise, fading, or audio quality.
Put it into practice
Listen before reporting, give the real readability and strength you observe, add tone for CW, and supplement the numbers with specific audio or interference comments when useful.
Practical boundary
Use current equipment instructions, battery and controller limits, measurement references, operating rules, and observed propagation. Planning values and propagation labels are not guarantees.
