NSA Exceptionally Controlled Information

ECI is a classification above Top Secret. It's for things that are so sensitive they're basically not written down, like the names of companies whose cryptography has been deliberately weakened by the NSA, or the names of agents who have infiltrated foreign IT companies.
As part of the Intercept story on the NSA's using agents to infiltrate foreign companies and networks, it published a list of ECI compartments. It's just a list of code names and three-letter abbreviations, along with the group inside the NSA that is responsible for them. The descriptions of what they all mean would never be in a computer file, so it's only of value to those of us who like code names.
This designation is why there have been no documents in the Snowden archive listing specific company names. They're all referred to by these ECI code names.