May be "X" refers to the Pinyin of "芯", which means the "core" stage if you put it in the chinese rocketry context. Classic "Y" means the production number of the rocket was given after it leaves the factory. Since the CZ-10B 1st stage could be reusable , they may be need a new name to refer a used 1st stage. I am guessing may be in the future when they starts reuse the 1st stage, the rocket name could be CZ-10B X1Y2, X1 means the No 1 reused 1st stage, Y2 means the expendable 2nd stage or so.Also his suggestion of scheme changing "Y~ to X~" is likely another mistake. Letter Y is used because it is the first letter in Pinyin of 遥, not because Y has any meaning in Chinese language. The launch would be disignated as 长征十号乙遥一. X is not initial letter of any Chinese words that may be related to launch sequence in my knowledge. In Chinese writting latin letter X is a place holder for "something" when for some reason the "something" is to be obscured






