A command paper is a document issued by the British government and presented to Parliament. White papers, green papers, treaties, reports from Royal Commissions and various government bodies can all be released as command papers, so called because they are presented to Parliament formally 'By Her Majesty's Command'.
Command papers are numbered. Since 1870 they have been prefixed with an abbreviation of 'command' which has changed over time to allow for new sequences.
Command Papers are published by TSO (The Stationery Office) for OPSI (The Office of Public Sector Information).
References
Command paper Wikipedia(Text) CC BY-SA