The term (short for cross-transcending) is self-explanatory. It means that after leveling a non-transcendent class to 99 Base - 50 Job, it is possible to change into any Transcendent class after rebirth to High Novice.
For example: If you made a level 99/50 Wizard, and you feel that the class isn't suiting you, you may rebirth and become a High Novice and then proceed to make any new Transcendent Class (High Swordsman, High Merchant, etc.).
Summary: Novice -> First Job -> Class 2-1 / 2-2 -> High Novice -> (ANY) Transcendent First Job -> Transcendent Class 2-1 / 2-2
This doesn't apply for Extended Classes; ie Gun Slinger, Taekwon, Ninja, etc.