X is just the letter prefix identifying the series of ITU-T Recommendations related to “Data networks, open system communications and security.” You can see a

X is just the letter prefix identifying the series of ITU-T Recommendations related to “Data networks, open system communications and security.” You can see a list of all 25 series (“W” is not assigned) and download most of the documents (including Rec. ITU-T A.12, Identification and layout of ITU-T Recommendations, which defines the letters) on the ITU’s website: https://www.itu.int/pub/T-REC https://www.itu.int/pub/T-REC