An Acronym for ITU Telecommunication Standardization Sector, formerly known as CCITT. ITU is an Acronym for 
International Telecommunications Union.
The ITU-T is responsible for the X509 and H.323 standards in the computing world, as well as lots of things to do with POTS standards and interoperability.
ITU specifications are horrible to read, they refer all over the place to each other, and feel like they should be read by a computer. Thing is, many of them are designed to be parsed by an ASN.1 parser.
ITU-T standards currently in force are freely downloadable from here.
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