“Stiff” isn’t a standard measurement.
Two shafts labelled “Stiff” need not have the same stiffness. Golf has no universal flex standard behind those letters, so matching the label on your old club does not guarantee the same behaviour in a new one. The letter is a guide within a range, not a common unit of measurement.
What a fitter can measure that the label cannot tell you
Clubmakers can measure how quickly a clamped shaft oscillates after it is deflected. The result is expressed in cycles per minute, or CPM. Under comparable test conditions, a higher frequency indicates a stiffer assembly.
That gives a fitter something more useful than assuming every manufacturer means exactly the same thing by R, S or X. It can also help when replacing a broken shaft or comparing clubs intended to feel consistent through a set.
Frequency is still a measurement within a particular test, not a complete description of how a shaft bends and feels. Shaft design and geometry matter too. When comparing clubs, use the flex marking to narrow the possibilities, then assess the actual club and its results. The familiar letter alone cannot establish that you have recreated the club you liked.