the golfer's swing speed;
his transition move from the end of the backswing to the beginning of the downswing;
his swing tempo;
and the point at which he unhinges the wrist-cock on the downswing (called the "release").
Normally, the higher the swing speed, the more forceful the transition move to start the downswing, the faster the swing tempo and the later the release, the stiffer the shaft should be to fit the golfer. The reverse is true for more flexible shaft fitting.
But there is an old statement in shaft flex fitting that says, "It's not always how fast the golfer swings the club, but how the golfer swings the club fast." This means the swing moves of transition, tempo and release are more important for shaft flex selection than is the golfer's swing speed on its own.
For example, let's say Golfer A and Golfer B have the same swing speed. However, Golfer A has a very smooth transition and tempo, and unhinges the wrist-cock about halfway down to the ball. Meanwhile, Golfer B has a forceful transition, a faster tempo and holds the release until late in the downswing.
Despite their identical swing speeds, Golfer B most definitely should be fit into a much stiffer shaft than golfer A. The reason is that a forceful transition, a faster tempo and a late release apply much more bending force on the shaft for the same final swing speed than does a smooth transition, smooth tempo and mid-way release of the wrist-cock.