A List X site is a commercial site (i.e. non-government) on UK soil that is approved to hold UK government protectively marked information marked as 'Secret' or above, or international partners information classified ‘Confidential’ or above.
A List X site is a commercial website located in the United Kingdom that has been approved to hold UK government data marked as "Secret" or higher, or information from international partners classified as "Confidential" or higher.