I think that transitional ballistics should be just evaluated as part of the external ballistics of the projectile. With that, I do understand that there may be niche applications of the evaluation of a projectile's ballistics upon exiting the barrel of the weapon, and those may need to be addressed by a bullet manufacturer or firearm producer, but I think it gets into the weeds a bit creating new names for each individual segment of a projectile's flight when a general understanding is what's needed in most cases. Transitional ballistics can certainly be a sub-category of external ballistics.

Polymer tips and VLD bullets have altered the trajectory of older cartridges in that they have increased the aerodynamic capabilities of the projectile and therefore increased stability in flight, reduced drag and increased distances of accuracy and kinetic energy carry-over when all other things remain the same. Increased aerodynamic capabilities also open the ability to potentially reduce a propellent load to get the same effect out of a cartridge.

Increased speed in flight, leading to less flight time and a more discernable flight path all can be correlated to improved projectile materials, projectile shape and balance. Although these aren't the only factors that affect flight, they play a large role and decreasing flight time can help to mitigate the environmental affects that can change the projectile's trajectory. Minimizing the bullet's profile in flight, but increasing it on impact (polymer tips) is a great way to ensure a fast, accurate flight, but also a deadly, forceful impact with maximal kinetic energy transfer into the target.