This is certainly and interesting conversation. RA is often discussed as a measure of stiffness and one racquet is compared to another in terms of its RA but I can design two racquets with the same frame shape, string pattern, weight, balance, swingweight and
RA, that can play quite differently. RA is just a measurement of the overall racquet deflection under load, with the less overall deflection at the tip producing a higher RA. However much like we all accept that there can be two racquets of 300g with massively different mass distribution (balance, swingweight, PMOI etc.) we can also have very different stiffness profiles for the same RA. In fact we measure the localized deformation under load at various different stiffness zones along the length of the racquet, for example the Phantom 100P and Phantom 100X 305 have the same RA, however the stiffness profiles are different:
Phantom 100P
| Zone | Deflection |
| I | 70 |
| II | 63 |
| III | 63 |
| IV | 49 |
| TORSION | 148 |
Phantom 100X
| Zone | Deflection |
| I | 82 |
| II | 69 |
| III | 52 |
| IV | 61 |
| TORSION | 183 |
Note here that these numbers are deflection, so the higher the number the more flexible it is in that zone (opposite to RA). Zone I is close to the tip, zone II is middle of hoop, Zone III is lower hoop and top of shaft, zone IV is lower shaft and top of handle. So in this example the Phantom 100X is actually stiffer in the lower hoop and shaft than the Phantom 100P but softer higher in the hoop, however they still have the same overall RA. Then we still have torsional stiffness values and in plane "twitch" stiffness which measure the stiffness of the hoop (imagine squeezing the frame between 3 and 9 or between 12 and 6 as examples, although we measure these in plane stiffness profiles across more zones also). These in plane stiffness also can make a dramatic difference to the response and feel of the frame.
All of this can be controlled through layup design, material choices, fiber angles etc. and we do tailor different racquets to have different stiffness profiles by design based on what we are trying to achieve in terms of racquet performance and feel.
I hope this helps, but I'm happy to elaborate on this if needed.
Tim
. We knew that we wanted to expand the range to include a 97" model and that starting with the 93P as the inspiration made more sense than the previous Phantom Pro 100. Additionally, when we embarked on the design of the new Phantom 100X design we realized that we needed a way to segment the line so that we could clearly identify the "boxy" style from the "round" style cross sections. With the success of the 93P and 100P, it made sense to just continue the "P" modifier to denote the boxy versions and then simply chose "X" as a companion modifier to describe the round versions. 
